Misc clean up before requesting focus via setFocusedWindow api

Replace focus changed callback to use the callback from InputDispatcher.
Currently the system posts a message and informs accessibility services,
window focus observers and window change listeners. With the exception
of accessibility service (which requires to be notified before the
client gets notified), the other callbacks should be synchronized with
the callback from InputDispatcher.

This change also cleans up null checks for ActivityRecord#appToken
since it cannot be null.

Test: presubmit
Bug: 151179149
Change-Id: Id79553835048c090505589d62dee9cb8f38f23ff