[Multi-user] Change ActivityManagerService to use userId.

1. The book-keeping needs to be per user.
2. The calls into IBackupManager need to pass in the userId.
3. convert clearPendingBackup to an internal service call.

Bug: 121197004

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: If49e00fc1d6aa770815c454f01b53865f6a68db4