Add View#cancelPendingInputEvents API

This API allows an application to cancel deferred high-level input
events already in flight. It forms one tool of several to help apps
debounce input events and prevent things like multiple startActivity
calls, FragmentTransactions, etc. from executing when only one was
desired since it's otherwise not desirable for things like click
events to fire synchronously.

Change-Id: I60b12cd5350898065f0019d616e24d779eb8cff9