DO NOT MERGE Split BackEvent into an internal BackMotionEvent and public BackEvent.

The constructor of BackEvent has diverged on QPR and master, which will
create merge conflicts for all subsequent SysUI back animations to be
added from QPR. This cherry-picks ag/20445076 minus the public API
changes to solve this problem.

Test: atest BackAnimationControllerTest
Test: atest BackNavigationControllerTests
Test: atest WindowOnBackInvokedDispatcherTest
Test: atest TouchTrackerTest
Test: m -j
Bug: 238475284
Change-Id: Ib9100a9d667a9a17e8f357a1bfc3ee2b52ec17c7