Scribe initiation without InputConnection

Allow Stylus handwriting to initiate handwriting without waiting for
InputConnection to be established.
When an editor view receives focus, it sends startInput binder call immidiately.
Focused editor (servedView) is then ready for any
startStylusHandwriting() calls.

This should help shave few millis from initiation latency.

Note: this intentionally doesn't optimize delegation since that is
migrating to newer APIs.

Bug: 308827131
Bug: 324670412

Test: atest HandwritingInitiatorTest StylusHandwritingTest

Change-Id: I0174e15fbbedf88dcd68ebe98333b6b07871985a