Scribe: Prevent scrolling when starting handwriting

After startStylusHandwriting is called, there is a short period before an ACTION_CANCEL motion event is dispatched. During this period, ACTION_MOVE motion events may continue to be received by the view root. This may cause a short scroll to occur before handwriting begins.

To prevent this, HandwritingInitiator will suppress ACTION_MOVE events
from a stylus gesture after startStylusHandwriting is called.

Fixes: 223805216
Fixes: 228210945
Bug: 222375667
Test: atest FrameworksCoreTests:android.view.stylus.HandwritingInitiatorTest
Test: atest CtsInputMethodTestCases:android.view.inputmethod.cts.StylusHandwritingTest
Change-Id: If7ed76f55c6a5f010631762733e851b5314334de