Always require opt-in for root/system

If a process with lower target SDK shares UID with the system the compat
change turns off the feature and potentially compromises the security of
the whole system.
There are no occurences in droidfood that would be affected (see
http://shortn/_bxSR7UaDa3).

Test: atest BackgroundActivityLaunchTests
Bug: 296479164
Flag: EXEMPT bugfix
Change-Id: I0ec16247ef80244afa5e351b9faef4b56dec61b5