Refactor acceptStylusHandwriting to use variant.

acceptStylusHandwriting(...) should use vairant with flags instead of
calling internal method directly.

We already have a test for acceptStylusHandwritingDelegation(..
, int flags, ..) in
StylusHandwritingTest#testHandwriting_delegateFromHomePackage,
this refactor makes the API test coverage happy.

Bug: 331548567
Test: atest StylusHandwritingTest
Change-Id: Ie5b4f127bc93cd839a155144368af500ed15447c