Remove batched consumption from onStop

This patch reverts the earlier patch, ag/11959351, because the correct
solution is aosp/1459888.

When batched input is available, the Choreographer::CALLBACK_INPUT is
scheduled by ViewRootImpl. The callback always runs, even if onStop has
been called. So the earlier patch was invalid.

In practice, what happened was that the batched input was not getting
consumed after FOCUS event was received in the receiver. This was fixed
in aosp/1459888.

The consumption mechanism is complex, so I added a doc explaining how it
works, for future readers.

The newly added test, IncompleteMotionTest, ensures that this regression
does not happen again.

Test: atest IncompleteMotionTest
Bug: 160561987
Change-Id: Ic0a453ebbce1f847de8e9cb87bdffdaf72c12637