Improve mouse handling in DragStartHelper

Ignoring ACTION_MOVE events not caused by an actual
mouse move (such as secondary button press).

Ensuring that OnDragStart is not called repeatedly
after it has returned true.

Adding tests for all major use cases.

Bug: 29570793
Test: android.support.v13.view.DragStartHelperTest
Change-Id: I457e8cd4f32ae6ad3f3b7de443e456e60ca755d5
6 files changed