Use signed in, signed out to describe whether the DB is open or closed.

Hopefully this gives users a better understanding of whether their
messages and contacts are accessible than "Briar is running" and "Quit".

briar-android/src/org/briarproject/android/BriarActivity.java

@@ -67,7 +67,7 @@ public class BriarActivity extends RoboFragmentActivity {
 		if(bound) unbindService(serviceConnection);
 	}
 
-	protected void quit() {
+	protected void signOut() {
 		new Thread() {
 			@Override
 			public void run() {