[Multi-user] Make backup dirs user-specific

1. For system user, functionality remains almost (see 2) exactly the
same.
2. Change the full backup dir which is used only to write temporary
manifest and meta dirs by the system process when doing full backup.
This is so that we dont have to worry unnecessarily about yet another dir.

Bug: 120424138

Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest $(find \
frameworks/base/services/tests/servicestests/src/com/android/server/backup \
-name '*Test.java')
3) atest CtsBackupTestCases
4) atest CtsBackupHostTestCases
5) atest GtsBackupTestCases
6) atest GtsBackupHostTestCases
7) 'adb shell bmgr' enabled/backupnow flow

Change-Id: I9a33547c9595a86b62869ee731d4c75a029922e8