Allow pinned activities to enter fullscreen using the api

Currently, the api assumes that the request is coming from the top most
focused activity. However, it may come from a pinned task, and, in that
case, that would not be true. This change removes that assumption by
passing in the task the activity is in rather than passing in the top
focused task.

Bug: 315352482
Test: Test using multiwindow fullscreen api test app (ag/20460053)
Change-Id: I86a9d7f3a8bebb8585f28343ab8623bce1bdaf4f