Enable FGS BG start restriction for apps targeting S

Also temporarily exempt Chrome, WebView and their variants, which
already all target S.

Bug: 175310205
Test: Boot, presublit
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/am/ActiveServicesTest.java
Change-Id: I5b7aeb3d2b37f410e8b7220d3c3996e2296437c8