Fix blocked event handling in InputDispatcher.

Bug: 5161854

The InputDispatcher contains code to recover from ANRs more quickly
when the user touches a different application.  When triggered,
it sets mNextUnblockedEvent to the next event it should dispatch
(to the next application).  All prior events are dropped.

This change fixes a bug where mNextUnblockedEvent could potentially
never be reset back to NULL if the inbound queue was drained
for some reason.

Change-Id: I21be2f02c9fc770bd260ea414db8bb184df89ada