Replaces AsyncTask with main handler for WindowMagnification

AsyncTask is deprecated so we use main handler instead to
return callback, onWindowMagnifierBoundsChanged.
And this change also add a null protection and unregister
OnLayoutChangeListener when window magnification is disabled.
And it can prevent null pointer exception.

Bug: 151394161
Bug: 151571627
Test: manual test, atest WindowMagnificationTest
    WindowMagnificationControllerTest
Change-Id: I421424582d8bad9e58fe5aca11feb75a31bd5b35