[Multi-user] Add -user param to adb backup/restore

Add an optional parameter -user to provide ID of the user for which to
run backup/restore operation. Add robolectric test to verify the
new parameter is proccessed correctly.

Bug: 119908153
Test: 1) atest BackupTest
      2) atest BackupManagerServiceTest
      3) atest TrampolineTest
      4) atest GtsBackupTestCases
      5) atest GtsBackupHostTestCases
      6) Manual:
        - Run "adb backup -all" and verify that backup is successfull
        - Run "adb restore" and verify that restore is successfull
        - Run "adb backup -all -user 10" and verify that backup faield as
          it's only currently supported for system user
        - Run "adb restore -user 10" and verify that restore failed as it's
          only currently supported for system user

Change-Id: I6dbf9c87eedd5a72da0446beff7d2551f98f2654