Ignore back invoke when window focus has lost

The back invoke target could lost focus during back navigation.
To prevent the non-focused window could still trigger back action cause
some unexpected behavior, this CL will listen focus changed and skip the
back invoke call.

Test: launch a trampoline activity and trigger back before next activity
shown.
Test: atest WindowOnBackInvokedDispatcherTest
Bug: 238050065

Change-Id: Ifd345b2283c5d07628e2884db6e4e13f3ec31e83