Fix erroneous band selection start.

Fix band selection to not start when crossing off of a grid item into empty space.
This CL also introduces a MotionEvent wrapper class, since MotionEvent
can't be used in tests.
Note that this CL works around several issues with b/23793622.

Bug: 23727363
Change-Id: I010a82db3363d99f2d804db2653a3a25d8cac940