mobile:  Deliver all touch events.

Android delivers events in a batch.  Previously, only the first event of the batch was passed to the Go callback, resulting in many events (typically corresponding to non-primary touches) being dropped.

Change-Id: I4b01bb6c4d1c88dec6c8fa379875043dfc0e9986
Reviewed-on: https://go-review.googlesource.com/1894
Reviewed-by: David Crawshaw <crawshaw@golang.org>