Return compat callback for apps requesting legacy back.

We don't need to check the enableOnBackInvokedCallback attribute value
because client side framework code will register the correct callback.

All apps need either the compat callback or default closing callback to
close.

Test: Swipe back on apps requesting legacy back. Then again on apps
using new back.
Test: atest frameworks/base/services/tests/wmtests/src/com/android/server/wm/BackNavigationControllerTests.java
Bug: 219850675

Change-Id: I57086febaf15d6ab04ab668a9a84623f4231c22f