Make HandwritingGesture methods final

Make getGestureType() & getFallbackText() final since we don't expect
these methods to be overriden by concrete gestures.
This shouldn't cause any user observable behavior change.

Bug: 256016481
Test: atest CtsInputMethodTestCases
Change-Id: I865bed67089b06a572f5f085cbe04d0538928cb3