Inject back key events if no focus window found.

If user applied a back gesture but core cannot find a focus window,
inject back key event, so that event can dispatch to client once it
gain focus window. And since the gesture was happened, treat it as
onBackPressed.
And if device is playing recents animation but the focus window is
point to live tile, we can also inject back key because Home/Recents
activity shall not have the same behavior as normal app.

Bug: 237629607
Test: cross test that back gesture can finish recents with legacy/shell
transition on Nexus/3rd launcher.
Test: test on BackTestApp for both 3btn mode and gesture mode.
Test: atest AnrTests BackNavigationLegacyGestureTest
Test: atest OnBackInvokedDispatcherTest KeyboardVisibilityControlTest
BackInvokedOnWidgetTest BackNavigationTests BackNavigationControllerTests

Change-Id: If754e66ea7c1c4ec313c45d17d7f6c10bfc2c465