Eliminate potential reentrance from unregisterInputChannel.

Ensure that all callbacks into the window manager policy occur
on the input dispatcher thread in the right place.  This fixes
a potential deadlock that may occur if the window manager
unregisters an input channel while holding its own lock.

The change is simply to defer running asynchronous commands
(usually callbacks into the policy) until the next iteration
of the dispatch looper thread.

Bug: 7382388
Change-Id: I90095580d717fcddb2209ef332df56400f837a34