Prevent CropView from getting confused by multiple pointers.

Not really implementing full multitouch, but ensuring that letting up on
the first finger doesn't just continue the event with another one.

Also refactor some of the MagnifierView's event listening to decouple it
from CropView a bit more.

Bug: 183240525
Test: Touch one edge with one finger, then put another one down and lift
      the first, ensure that movement doesn't continue with second finger.
Change-Id: Ia9f0293a914fb85f80a746f1093a3b23b23eab73