Make Letterbox.InputInterceptor thread safe

InputEventReceiver#finishInputEvent is invoked every time
a double-tap event occurs on the ui thread and so it cannot
be guarded by the mWmService.mGlobalLock.

To avoid race conditions the common practice is to invoke
the InputEventReceiver#dispose() on the same thread dispatching
input events.

Fix: 317428610
Test: atest WmTests:LetterboxTest

Change-Id: I4cbaa9ede668a4434dd59d6a7758bec961cc1eeb