Input dispatcher ANR handling enhancements.

This change is essentially a rewrite of the main input dispatcher loop
with the target identification folded in.  Since the input dispatcher now
has all of the window state, it can make better decisions about
when to ANR.

Added a .5 second deadline for processing app switch keys.  This behavior
predates Gingerbread but had not previously been ported.

Fixed some timing inaccuracies in the ANR accounting that could cause
applications to ANR sooner than they should have.

Added a mechanism for tracking key and motion events that have been
dispatched to a window so that appropriate cancelation events can be
synthesized when recovering from ANR.  This change helps to keep
applications in sync so they don't end up with stuck buttons upon
recovery from ANRs.

Added more comments to describe the tricky parts of PollLoop.

Change-Id: I13dffca27acb436fc383980db536abc4d8b9e6f1