CapturedTouchpadEventConverter: filter out palms

When a touchpad marks a touch as a palm, we don't want to pass it on to
apps that have captured the touchpad. In future, we could pass these
through if we wanted by removing @hide from MotionEvent.TOOL_TYPE_PALM
and using that to denote palm touches.

Bug: b/259547750
Test: atest inputflinger_tests
Test: on a suitable touchpad captured by a test app, move a palm around.
      Check it's not reported at all when the pad identifies it straight
      away, and that it's cancelled when identified after touching down.
      (`getevent -l | grep TOOL_TYPE` is useful to tell when the pad
      changes its classification of the touch)
Change-Id: Iefe84120321246f3661a4d2d06e0ec01ba9fe52b