Add an option to automatically lock the app

This adds an option to automatically lock the app when:
* The back button is pressed
* The device is locked

It's the first step towards implementing #7

app/src/main/java/com/beemdevelopment/aegis/AegisApplication.java

@@ -1,7 +1,10 @@
 package com.beemdevelopment.aegis;
 
 import android.app.Application;
+import android.content.BroadcastReceiver;
+import android.content.Context;
 import android.content.Intent;
+import android.content.IntentFilter;
 import android.content.pm.ShortcutInfo;
 import android.content.pm.ShortcutManager;
 import android.graphics.drawable.Icon;
@@ -24,6 +27,10 @@ public class AegisApplication extends Application {
         _manager = new DatabaseManager(this);
         _prefs = new Preferences(this);
 
+        // listen for SCREEN_OFF events
+        ScreenOffReceiver receiver = new ScreenOffReceiver();
+        registerReceiver(receiver, new IntentFilter(Intent.ACTION_SCREEN_OFF));
+
         if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.N_MR1) {
             initAppShortcuts();
         }
@@ -57,4 +64,20 @@ public class AegisApplication extends Application {
 
         shortcutManager.setDynamicShortcuts(Collections.singletonList(shortcut));
     }
+
+    public boolean isAutoLockEnabled() {
+        return _prefs.isAutoLockEnabled() && _manager.isEncryptionEnabled() && !_manager.isLocked();
+    }
+
+    private class ScreenOffReceiver extends BroadcastReceiver {
+        @Override
+        public void onReceive(Context context, Intent intent) {
+            if (isAutoLockEnabled()) {
+                _manager.lock();
+                Intent newIntent = new Intent(getApplicationContext(), MainActivity.class);
+                newIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
+                startActivity(newIntent);
+            }
+        }
+    }
 }

app/src/main/java/com/beemdevelopment/aegis/Preferences.java

@@ -27,6 +27,10 @@ public class Preferences {
         return _prefs.getBoolean("pref_intro", false);
     }
 
+    public boolean isAutoLockEnabled() {
+        return _prefs.getBoolean("pref_auto_lock", true);
+    }
+
     public void setIntroDone(boolean done) {
         _prefs.edit().putBoolean("pref_intro", done).apply();
     }

app/src/main/java/com/beemdevelopment/aegis/ui/MainActivity.java

@@ -363,6 +363,15 @@ public class MainActivity extends AegisActivity implements EntryListView.Listene
         updateLockIcon();
     }
 
+    @Override
+    public void onBackPressed() {
+        if (_app.isAutoLockEnabled()) {
+            lockDatabase();
+        }
+
+        super.onBackPressed();
+    }
+
     private BottomSheetDialog createBottomSheet(final DatabaseEntry entry) {
         BottomSheetDialog dialog = new BottomSheetDialog(this);
         dialog.setContentView(R.layout.bottom_sheet_edit_entry);
@@ -428,6 +437,7 @@ public class MainActivity extends AegisActivity implements EntryListView.Listene
                 return true;
             case R.id.action_lock:
                 lockDatabase();
+                startAuthActivity();
                 return true;
             default:
                 if (item.getGroupId() == R.id.action_filter_group) {
@@ -475,7 +485,6 @@ public class MainActivity extends AegisActivity implements EntryListView.Listene
             _entryListView.clearEntries();
             _db.lock();
             _loaded = false;
-            startAuthActivity();
         }
     }
 

app/src/main/res/values/strings.xml

@@ -30,6 +30,8 @@
     <string name="pref_tap_to_reveal_title">Tap to reveal</string>
     <string name="pref_tap_to_reveal_summary">Tokens will be hidden by default. Tap on the tokens to reveal code.</string>
     <string name="pref_tap_to_reveal_time_title">Timeout for tap to reveal</string>
+    <string name="pref_auto_lock_title">Auto lock</string>
+    <string name="pref_auto_lock_summary">Automatically lock when you close the app or lock your device.</string>
     <string name="pref_encryption_title">Encryption</string>
     <string name="pref_encryption_summary">Encrypt the database and unlock it with a password or fingerprint</string>
     <string name="pref_fingerprint_title">Fingerprint</string>

app/src/main/res/xml/preferences.xml

@@ -42,13 +42,18 @@
             android:title="@string/pref_secure_screen_title"
             android:summary="@string/pref_secure_screen_summary"
             app:iconSpaceReserved="false"/>
+        <androidx.preference.SwitchPreferenceCompat
+            android:defaultValue="true"
+            android:key="pref_auto_lock"
+            android:title="@string/pref_auto_lock_title"
+            android:summary="@string/pref_auto_lock_summary"
+            app:iconSpaceReserved="false"/>
         <androidx.preference.SwitchPreferenceCompat
             android:defaultValue="false"
             android:key="pref_tap_to_reveal"
             android:title="@string/pref_tap_to_reveal_title"
             android:summary="@string/pref_tap_to_reveal_summary"
             app:iconSpaceReserved="false"/>
-
         <Preference
             android:key="pref_tap_to_reveal_time"
             android:title="@string/pref_tap_to_reveal_time_title"