Forward back key events when no top onBackInvokedCallback registered.

This allows the event to propagate through the view hierarchy, for
instance, from a floating window that has not registered its own callback.

Bug: 415970831
Flag: EXEMPT bugfix
Test: create floating window by using FrameLayout, verify it can
receive back key even if no onBackInvokedCallback registered.

Change-Id: I106ba93d4a793a634535d2dcbbd4f145d7a3a4f0