Adapt HandwritingInitator test for Scribe pref

Update the HandwritingInitiator test so that it adpats to new
definition in iStylusHandwritingAvailable() which now also checks if
Scribe user preference is enabled.

Bug: 228514000
Test: atest FrameworksCoreTests:android.view.stylus.HandwritingInitiatorTest
Test: atest StylusHandwritingTest
Change-Id: I84375568e16555e3563d1a1d10832c7e36ed5d42