[Multi-user] Make each user have their own backup thread

The original backup thread is now only used by Trampoline and BMS for
general non-user specific operations. Each UserBMS instance has its own
'backup-[user id]' thread to allow for parallelism between users. When a
user is stopped, we quit this thread.

Bug: 121305979
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) Start system user -> verify backup-0 thread created;
Start secondary user -> verify backup-10 thread created;
Stop secondary user -> verify backup-10 thread stopped;
Verify work posted to the thread is executed (register transports, bmgr
backupnow, etc.)

Change-Id: Iaa37fb3e2eb442653e27565ca13a88a0582efa9d