[flexiglass] Navigate away from bouncer when the IME is hidden.

Note that we need to wait for the IME to show before we handle its
dismissal or we risk a race condition where the logic that checks if the
IME is closed runs before the IME gets a chance to be shown.

Fix: 299343836
Test: added unit tests
Test: manually verified that dismissing the keyboard in the password
bouncer also moves back to the lockscreen scene. Done with 3 button
navigation and with gesture-based navigation.
Test: manually verified that entering the correct password works to
unlock the device.

Change-Id: I77e8dd32abe776f9df42c4fd757de3d889df9bb2