Prevent handling touchpad gesture on desktop window decorations

When a user is performing a touchpad gesture, e.g. two finger swipe,
MotionEvent contains classification tag. When there's a such tag, we
should not respond to the synthesized finger movements.

Bug: 393330032
Test: DesktopModeWindowDecorViewModelTests
Test: DragDetectorTest
Test: Manually try touchpad gesture on window edges and caption bar.
Flag: EXEMPT bug fix
Change-Id: I93fa5684562a2b4ee8c0eed7811a868f0e7bd29f