Dispose HideNavInputEventReceiver on PhoneWindowManager's handler

Disposal of input event receiver needs to happen on the handler
specified for receiving the inputs, otherwise nativeFinishInputEvent
could crash due to race with native dispose.

Instead of disposing mInputConsumer in beginLayoutLw(), post to
PhoneWindowManager's handler to dispose, and skip any input events
received after mInputConsumer is set to null by beginLayoutLw().

bug: 26927018
Change-Id: I094eb4472ea68f2c8bd6a428161d7edb11dc8900