Optimize dispatcher for back-to-back finished signals.

Minor tweak to the dispatcher to handle as many finished signals
in a receive callback as possible instead of going back to
the Looper and waiting for the next poll() to hit the callback
again.

This is part of a series of changes to improve input system pipelining.

Bug: 5963420

Change-Id: I8471107371693e21ce8ce7cca1e8d79ba4ca2351