[Multi-user] Convert key-value flow to use user id

Change paths that key-value backup/restore flow goes through to use
user id.

Bug: 121198605
Bug: 121198606
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest $(find \
frameworks/base/services/tests/servicestests/src/com/android/server/backup \
-name '*Test.java')
3) adb shell bmgr backupnow [kv package]
Change-Id: Ie192391561f8e03f623c4d86914b3787ff2c0f88