Improve Launcher drag performance.

Launcher swiping looks choppy. It's because we deliver the
motion events of the drags asynchronously, and sometimes we
may get an event to redraw before the motion event is posted, so we
end up drawing again without updating to the latest motion
information.

This fix makes input event processing more proactive. Every time
we run ViewRootImpl.performTraversals() (which is what
happens whenever we need to layout, measure, and/or draw), we
first process all pending input events, ensuring that we are
completely up-to-date with posted events prior to drawing, so that the
drawing we do is synchronous with the events we've received.
This eliminates the choppiness and means that we can now get the full
refresh rate on the screen with drag events.

The fix was done for Launcher, but it is pervasive in the system,
so this may fix other laggy drag behavior as well.

Change-Id: I8dbed6acadc2662f317f736e769f536f555701aa