Fix handwriting support on fake editors using delegation

We originally added support for Scribe on fake editors in bug 228100684.
However, it doesn't address special cases like when real editor is a
different application package.

This CL introduces IMF APIs to suport handwriting delegation across different
view trees (with or within the same package).

Bug: 266834695
Bug: 228100684
Test: atest StylusHandwritingTest

Change-Id: I3a5d5dc574c5f78697f2a43e11729febf21cc4e2