Add an input feature for a stylus interceptor

A stylus interceptor is a window that receives all stylus events within
its touchable bounds, while letting all other events be dispatched to
windows behind it. This makes it possible for the framework to create a
stylus interceptor on top of another app to implement handwriting
recognition.

The feature has no effect when the window flag FLAG_NOT_TOUCHABLE is
not set.

The feature has no effect when the window is not a trusted overlay.

Test: manual
Change-Id: Ia481ee8d831643bc99445b47659a7f5e9875c388