Synthetic cancel event should have touchscreen source.

Fixed a warning identified by the InputEventConsistencyVerifier
due to some code that synthesizes a touch event which it sends
to onTouchEvent with no input device source specified.
This is generally harmless but a fake source is better than
none in this case.

Change-Id: I44cfa36a4746f598f8fde3086084af3896e2ba2a