[Multi-user] adb fullbackup: support userId

Bug: 121198030

Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest $(find \
frameworks/base/services/tests/servicestests/src/com/android/server/backup \
-name '*Test.java')
3) atest CtsBackupTestCases CtsBackupHostTestCases GtsBackupTestCases GtsBackupHostTestCases
4) tested "adb backup -all -obb -apk". confirmed that log contains "obb dir: /storage/emu.." etc. restore of wallpaper worked.

Change-Id: Ic8ff71c5dd949aecc7561a3bac5033e822fcf305