add option --user to Bmgr.java

Bug: 118480337

Test: 1) atest RunFrameworksServicesRoboTests
    2) atest BmgrTest
    3) atest CtsBackupTestCases
    4) atest CtsBackupHostTestCases
    5) atest GtsBackupTestCases
    6) atest GtsBackupHostTestCases
    UsageStatsRestoreHostSideTest fails. I verified locally that this test
    fails without this change too.
    7) flashed device and ran:adb shell bmgr --user 10 backupnow --all
    Logs and output are as expected since backupservice isnt active for user 10
    log: Bmgr    : Running backupnow for user:10
    output: Error: Could not access the Backup Manager.  Is the system running

    8) flashed device and ran:adb shell bmgr backupnow --all
    log: Bmgr    : Running backupnow for user:0
    output:
	Running incremental backup for all packages.
	Package @pm@ with result: Success
	....
	Backup finished with result: Success

Change-Id: Icfebe26f36276ea687d15c9f9c361c409313ae9d