Show a backup reminder if auto backups are not enabled

If the user doesn't have auto backups enabled, the reminder will pop up
up every time a significant change is made to the vault.

Users can get rid of the reminder by:
- Creating an export
- Enabling automatic backups (either ours or Android backups will do)

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

@@ -308,6 +308,16 @@ public class Preferences {
         return _prefs.getString("pref_backups_error", null);
     }
 
+    public void setIsBackupReminderNeeded(boolean needed) {
+        if (isBackupsReminderNeeded() != needed) {
+            _prefs.edit().putBoolean("pref_backups_reminder_needed", needed).apply();
+        }
+    }
+
+    public boolean isBackupsReminderNeeded() {
+        return _prefs.getBoolean("pref_backups_reminder_needed", false);
+    }
+
     public boolean isPinKeyboardEnabled() {
         return _prefs.getBoolean("pref_pin_keyboard", false);
     }