Introduce developer option for Stylus handwriting

Introduce a new developer option for Input Method:
Stylus handwriting: when this setting is enabled, currently selected
input method receives stylus events after focus on editor.

Bug: 219777744
Test: atest StylusHandwritingPreferenceControllerTest
Change-Id: Ia6335df60aae56973ed54bec1a243ea1de3aa6ad