Add support for VR InputMethod.

This change adds support for VR-only IMEs in InputMethod framework.
In order to set this VR IME, setVrInputMethod(ComponentName) should be
called by VrManager.

When VrManager calls setVrInputMethod(), IMMS changes updates
the selected input method in a transient way i.e. it doesn't
update the Settings or input history. Once VR mode finishes,
it restores last input from settings.

Bug: 63037786
Test: Manually using the sample app in bug.

Change-Id: I1db7981b5198e7e203d4578cae7e5b6d20037d0d