A11yInputFilter: only multiplex pointer events

For events that are focus-dispatched, like joystick events, and also for
ACTION_SCROLL events (since they are always treated independently), we
don't need to turn on the multiplexer. The multiplexer should only be
used on pointer events.

The AccessibilityDisplayProxyTest was tested on tangor by first removing
all of the test assumptions locally (otherwise, the tests were skipped
due to assumption failure).

Bug: 326686780
Test: atest AccessibilityInputFilterInputTest
Test: atest android.accessibilityservice.cts.AccessibilityEndToEndTest#testOnMotionEvent_interceptsEventFromRequestedSource_SetAndUnset
Test: atest AccessibilityDisplayProxyTest
Change-Id: I40a1b42aa8a169138c96972a39ebed5d16483d11