Make PointerLocationView more resilient.

Right now it crashes if we miss an up event since the active pointer
won't get reset. We can't just clear this in onDetachedFromWindow
because of how it gets drawn in system_server, so instead we just check
before using the active pointer.

Bug: 109779280
Test: put settings into split screen, frequently enable / disable
      Pointer Location while tapping in another app window. See that it
      doesn't crash.
Change-Id: If2b1f3157574c962b24115b0ecf0a27feec8e84c