Fix app lock after sign-out bug

It some cases, it was observered how the app was locked after the user
had signed out.
This commit ensures that set alarms are canceled and that no new ones
will be set after the LockManager service has been stopped.

Fixes #1386

briar-android/src/main/java/org/briarproject/briar/android/account/LockManagerImpl.java

@@ -101,6 +101,8 @@ public class LockManagerImpl implements LockManager, Service, EventListener {
 
 	@Override
 	public void stopService() {
+		timeoutMinutes = timeoutNever;
+		if (alarmSet) alarmManager.cancel(lockIntent);
 	}
 
 	@UiThread