Clear handwriting delegation once its started

Handwriting delegation can fail to start if stylus stroke on delegate is
short. This happens because IMMS.startStylusHandwriting clears delegation early
and an immediate ACTION_UP can stop recording gesture before
HandwritingModeController gets startHandwriting signal.

We should clear delegation once handwriting has started.

Bug: 293305048
Test: StylusHandwritingTest
Change-Id: I37b90ff0a236aa8670972f5d8d77fbecbe65a5f2