Dispatch multiple touch events in parallel.

This change enables the input dispatcher to send multiple touch
events to an application without waiting for them to be acknowledged.
Essentially this is a variation on the old streaming optimization
but it is much more comprehensive.  Event dispatch will stall as
soon as 0.5sec of unacknowledged events are accumulated or a
focused event (such as a key event) needs to be delivered.

Streaming input events makes a tremendous difference in application
performance.  The next step will be to enable batching of input
events on the client side once again.

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

Bug: 5963420
Change-Id: I025df90c06165d719fcca7f63eed322a5cce4a78