Add support for the PointerLocation overlay.

This change involves adding a new method to IWindowManager,
monitorInput() that returns an InputChannel to receive a copy of all
input that is dispatched to applications.  The caller must have
the READ_INPUT_STATE permission to make this request (similar to
other window manager methods such as getKeycodeState).

Change-Id: Icd14d810174a5b2928671ef16de73af88302aea0