Force embedded activity resizable

Assume the host task should be resizable so the embedded
activities belonging to the task should also be resizable.

Because an activity may not want to be put in system split
screen but allow to be embedded by a resizable task. This
change make the activity fit the bounds of the task fragment
instead of being letterboxed.

Bug: 199581491
Test: Launch a unresizable activity to embedded split.
Change-Id: Ibe0c41cc27de4d285f4e5ebd2fd75ed13ef96fbc