Scribe in View: Introduce HandwritingInitiator

This CL only implement the handwriting initiation for milestone 1:
a) handwriting is only initiated for a focused editor.
b) user must start handwriting within the editor's boundary.

Bug: 209854913
Test: atest FrameworksCoreTests:HandwritingInitiatorTest
Change-Id: Iab6d83f58aa74f0af8091731c80bbdb2eb29c024