Hold a weak reference to PointerController when handling vsync

Currently PointerController starts listening to display events
immediately (in its constructor) and never explicitly removes
the callback. The reference dangling from the looper
prevents the PointerController instance from being deleted
when all the clients have released their references.

As a result, when USB or BT mouse is disconnected,
the mouse stays frozen on screen and only goes away
after a 15 sec inactivity timeout.

This change introduces an intermediary LooperCallback
which holds only a weak reference to PointerController.
The pointer now disappears immediately upon mouse
disconnect.

Bug: 30824220
Change-Id: I5f7208dbfa381b3e21f248cc0da402f307faa184