Conditional opting of system into BAL enforcement

Adding a flag to control whether background activity launching
restrictions are enforced for system activities, or not.

This avoids enforcement based on the target-sdk-level, which is
different for the system between user and userdebug builds.

Ignore-AOSP-First: Internal changes, flags file incorrectly marked as aosp-first.
Bug: 339403750
Test: atest CtsWindowManagerBackgroundActivityTestCases:android.server.wm.ActivitySecurityModelTest

Change-Id: Ie2c6949047194adfce01747fe7c1af132fccefc8