Better handling of multiple touch events in GlowPadView

This fixes a bug where a secondary touch event from the edge of the screen
would cause GlowPadView to choose the wrong target.  The issue is resolved
by keeping track of pointer ids and only allowing the one that initiated
the gesture to complete it.

Fixes bug 7133500

Change-Id: If296b60af2421bfa1a9a082e608ba77b2392a218