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-api/src/main/java/org/briarproject/bramble/api/account/AccountManager.java

@@ -67,4 +67,8 @@ public interface AccountManager {
 	 * and stored.
 	 */
 	boolean changePassword(String oldPassword, String newPassword);
+
+	boolean isLocked();
+
+	void setLocked(boolean locked);
 }