Rename "Database" to "Vault"

We decided on calling the state file the "vault" a while back. This patch makes
the naming consistent across the codebase. I left "DatabaseImporter" classes
alone, because I'm not sure what a better name for those would be.

app/src/main/java/com/beemdevelopment/aegis/vault/Vault.java

@@ -0,0 +1,56 @@
+package com.beemdevelopment.aegis.vault;
+
+import com.beemdevelopment.aegis.encoding.Base64Exception;
+import com.beemdevelopment.aegis.otp.OtpInfoException;
+import com.beemdevelopment.aegis.util.UUIDMap;
+
+import org.json.JSONArray;
+import org.json.JSONException;
+import org.json.JSONObject;
+
+public class Vault {
+    private static final int VERSION = 1;
+    private UUIDMap<VaultEntry> _entries = new UUIDMap<>();
+
+    public JSONObject toJson() {
+        try {
+            JSONArray array = new JSONArray();
+            for (VaultEntry e : _entries) {
+                array.put(e.toJson());
+            }
+
+            JSONObject obj = new JSONObject();
+            obj.put("version", VERSION);
+            obj.put("entries", array);
+            return obj;
+        } catch (JSONException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    public static Vault fromJson(JSONObject obj) throws VaultException {
+        Vault vault = new Vault();
+        UUIDMap<VaultEntry> entries = vault.getEntries();
+
+        try {
+            int ver = obj.getInt("version");
+            if (ver != VERSION) {
+                throw new VaultException("Unsupported version");
+            }
+
+            JSONArray array = obj.getJSONArray("entries");
+            for (int i = 0; i < array.length(); i++) {
+                VaultEntry entry = VaultEntry.fromJson(array.getJSONObject(i));
+                entries.add(entry);
+            }
+        } catch (Base64Exception | OtpInfoException | JSONException e) {
+            throw new VaultException(e);
+        }
+
+        return vault;
+    }
+
+    public UUIDMap<VaultEntry> getEntries() {
+        return _entries;
+    }
+}