Infra for SysUI to monitor FGSes

There's now a way for it to track what apps currently have services in
the FGS state in real time.

Bug: 192504071
Test: manual
Test: atest CtsAppTestCases:android.app.cts.ServiceTest
Ignore-AOSP-First: new API should not be revealed in AOSP prior to drop
Change-Id: Ief1b6f8ef7deba6abbbf42f20b3f498861c1fe69