[Multi-user] Create setting for multi-user backup service support

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