Fix forceHiddenForPip for freeform mode

The windowing mode set to exit PiP isn't necessarily UNDEFINED, but
could be FREEFORM, FULLSCREEN, etc depending on the Shell's
policies.

Bug: 265106229
Test: Manually dismiss PiP and confirm it's not moved to front.
Change-Id: I9a6dd4d967f4e70bd56f38f16763a5ea37cee4cf