PriorityNestedScrollConnection simplification: remove onPostFling method

To simplify the logic for acquiring priority during postFling, we've
added a canStartPostFling method. If this method returns true, it will
call onStart and then immediately onStop (since the last event was
launched during a nested scroll gesture).

Test: atest PriorityNestedScrollConnectionTest
Bug: 291025415
Change-Id: I7ade358f2097a15d1328f3e6706df0ec12d23acf