Full teardown when the user stops background apps

Not just moving its services out of the FGS mode.

The shell command for invoking this operation has changed; it is now

    adb shell cmd activity stop-app [--user NUM] PACKAGE

Bug: 192504071
Test: manual (Maps nav, YouTube, etc)
Test: atest CtsAppTestCases:android.app.cts.ServiceTest
Test: atest RunningFgsControllerTest
Change-Id: Ic14c1ba557460e25dd1ba2fd79462918e9a47377