Promote DISALLOW_RUN_IN_BACKGROUND to SystemAPI

This already existed from some earlier work and has actually worked on
AOSP releases since Marshmallow if set with DevicePolicyManager or adb.
However it's been hidden all this time.

Bug: 71626497
Test: make update-api -j
Change-Id: I97453093ff5c35b7ffcd56e10cc8ab3fc3b843b7