Ensure handwriting session can only be started from inside app bounds

Handwriting should only be started if there is an ongoing stylus gesture
targeted at the focused app. If there is no ongoing stylus gesture on
the focused app, the startStylusHandwriting request will fail.

Update the StylusHandwritingTests for these new requirements.

Bug: 226081811
Test: atest StylusHandwritingTest
Test: manual with HandwritingIme test app
Change-Id: I9cc202a932cc7b79b4c0b0703ba9a3a92465b588
2 files changed