PointerChoreographer: Starting stylus hover gesture should fade mouse

This is a small UX improvement that fades the mouse when the stylus
starts hovering to avoid showing multiple cursors on the screen when the
mouse isn't used.

Bug: 335849698
Flag: EXEMPT bug fix
Test: Presbumit
Change-Id: I5849a4e4a8a41201e1e5fd1164214226142625b5