[RESTRICT AUTOMERGE] Attempt to fix exception in IMS

IMS might throw IllegalStateException in
WindowContextController#attachToWindowToken when #initializeInternal
is called. A possible root cause may be #initializeInternal is called
after onDestroy, which detach IMS from ImeContainer.

This CL add a flag in #onDestroy to prevent #initializeInternal from
being called after #onDestroy.

Bug: 211062619
Test: presubmit

Change-Id: Ie7814da801878a3487123fefdc9e71d0e1ed28d7