Whether the backup service supports multi-user is now configured in a
Global setting: backup_multi_user_enabled
This allows us to develop multi-user support hidden behind a flag. In a
future CL, we'll also gate the types of users we support.
Also create basic infrastructure for starting the service for a newly
unlocked user (currently a no-op).
Bug: 120212806
Test: 1) atest TrampolineTest
2) adb shell settings put global backup_multi_user_enabled 0;
unlock system user -> verify service started;
unlock user 10 -> verify service not started;
3) adb shell settings put global backup_multi_user_enabled 1;
unlock system user -> verify service started;
unlock user 10 -> verify service started;
Change-Id: I048e017cfa6148097cebe2eb2916d1b53c53d3b0
a6d828755e