[Multi-user] Pass userId on backup agent creation

Bug: 123349308
Test: 1) atest RunBackupFrameworksServicesRoboTests
         2) atest CtsBackupTestCases
         3) atest CtsBackupHostTestCases
         4) atest GtsBackupTestCases
         5) atest GtsBackupHostTestCases
         6) Manual: Go through kv/full backup/restore for system/secondary users
	    and verify that userId is passed correctly to the backup agents.

Change-Id: Iea747394163d9e473bce4fea890a01e2e4bfefbd

Mostly the same as the older ag/5267584 with the same purpose which didn't get
merget.