Make handwriting surface ignore touch when stylus is down

With support of simultaneous touch and stylus feature, we now allow
user to simulataneous touch and stylus on different windows. However,
stylus handwriting is a special case (using spy surface) can receive
stylus and finger touch in the app, which can steal focus and end stylus
handwriting session abruptly.

This change makes the app and system areas untouchable while stylus is
down to prevent accidental touches. When stylus is UP, its reverted to
NOT_TOUCHABLE (i.e. lets the touch passthrough).

Bug: 332850063
Test: atest StylusHandwritingTest

Change-Id: I927cee4e8ebb727f157d24b81558ccbe55696d14