Improve HandwritingInitiator handling of disabled views

Disabled views should still be tracked and considered for the purpose of
finding the best candidate view. However if the best candidate view is
disabled, then the handwriting initiator shouldn't take any further
action. Before this change, the handwriting initiator would try to
request focus which would fail, or try to start handwriting which would
fail.

Test: atest HandwritingInitiatorTest
Change-Id: I69a66dcf702bed54714bb09314fad8700ec510f2