Trigger app transition explicitly when nothing gets resumed

Currently, even if a freeform task gets moved back, the app
transition doens't start until timeout happens because usually
resuming a top activity triggers OPENING/CLOSING transactions, but
nothing gets newly resumed in this case.

Bug: 157104277
Test: Minimizing a freeform window works without timeout.
Change-Id: I420909156a72a191f1b22dc41ea0b350d91fb2fb