Remove exception for injecting pointer event from sendTrackballEventSync

Starting from T, a MotionEvent's default source is SOURCE_CLASS_POINTER.
This means injecting a newly obtained MotionEvent into
sendTrackballEventSync would cause the exception to be triggered.

Adding such an exception doesn't make sense in T, since it adds the
additional precondition to sendTrackballEventSync that the MotionEvent's
source must correctly configured. This precondition did not exist before,
so we remove the exception.

Bug: 228194124
Test: atest android.app.cts.DialogTest
Change-Id: I039e30e6c768ce11c64d077d20373da12f27b72f