Attemp handle back key when enable onBackInvoked callback.

Unlike normal activity, recents would receive key event from input
consumer instead of go throught ViewRootImpl, so when adapt to
predictive back API, recents should try to handle back key when receive
key event.

Flag: ACONFIG launcher.enable_predictive_back_gesture ENABLED
Bug: 333428882
Test: switch to 3-button mode, go to live tile, verify back key can
dismiss recents.

Change-Id: Ibe6d9b2475a0b89b12dc4b34251a2a92926b5a4e