Create "android" backup agent in the system process for all users

1) Return the system user's appInfo for "android" so we launch into the
system process.
2) In the system process, each user has its own instance of the "android"
backup agent to allow for parallel backups across users.
3) Update agent bookkeeping to key by user id.

Bug: 111500434
Test: 1) "adb shell bmgr backupnow android" for system and non-system
user; verify through debugger and logging that each user has its own
agent instance and both run in the system process; verify backup
succeeds
2) Repeat with "adb shell bmgr restore [token] android"

Change-Id: Ie9c8934daac02fb1c09f37753ef02ef957d18a75