MVP of PIN lock feature

This only shows the settings if a screen lock is available.
If the setting is activated, it shows a drawer item for locking.
Once locked, the screen lock secret needs to be entered to enter the
app again.

First part of #1247

bramble-core/src/main/java/org/briarproject/bramble/account/AccountManagerImpl.java

@@ -45,6 +45,7 @@ class AccountManagerImpl implements AccountManager {
 
 	@Nullable
 	private volatile SecretKey databaseKey = null;
+	private volatile boolean locked = false;
 
 	@Inject
 	AccountManagerImpl(DatabaseConfig databaseConfig, CryptoComponent crypto,
@@ -218,4 +219,16 @@ class AccountManagerImpl implements AccountManager {
 			return key != null && encryptAndStoreDatabaseKey(key, newPassword);
 		}
 	}
+
+	@Override
+	public boolean isLocked() {
+		return locked;
+	}
+
+	@Override
+	public void setLocked(boolean locked) {
+		synchronized (stateChangeLock) {
+			this.locked = locked;
+		}
+	}
 }