[Multi-user] No-op refactor to move backup dirs and settings, which should be user-specific,

to one class for each.

This is to be able to see all filesystem access by backup/restore
together so as to prepare to make all these directories be per user.

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) 'adb shell bmgr' enabled/backupnow flow

Change-Id: I9f30d22b9f609eaaf06d9e5a6fa5879ed1f0e653