Allow certain background restricted apps to run FGS

Previously background restricted apps are not allowed to run FGS
unless they're on the top.  Now if they're bound by top apps,
or they're bound by another app with FGS but without background
restriction, we still allow them to run FGS.

Bug: 187801547
Test: atest CtsAppTestCases:ActivityManagerFgsBgStartTest
Change-Id: I9c7b565275e33dae4383d188e678de880704af00