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