Dump information for all users running backup in backup dumpsys.

Currently backup dumpsys in bugreport only contains information
for system user making it hard to debug backup bugs in non-system
users. Therefore, add dumpsys information for all users running backup.

For system user, keep the dumpsys format the same for compatibility with
cts and gts tests. For non-system users, add a prefix "User <userid>:"
to all dumpsys headers.

The changes in Android.bp and AndroidManifest.xml are to support mocking
of the static method DumpUtils.checkDumpAndUsageStatsPermission in the
test testDump_systemUserFirst

Bug: 143867387

Test: atest com.android.server.backup.BackupManagerServiceTest

Test: atest com.android.server.backup.UserBackupManagerServiceTest

Test: adb shell pm create-user test1 -> say this gives 11
adb shell am start-user 11
adb shell bmgr --user 11 activate true
adb shell dumpsys backup users -> "Backup Manager is running for users: 0 11"
adb shell dumpsys backup -> contains both
"Backup Manager is enabled / setup complete / not pending init" and
"User 11:Backup Manager is disabled / not setup complete / not pending init"
adb shell pm remove-user 11

Test: "adb bugreport" on device with secondary user as created above and check that result
contains dumpsys for both system and secondary users.

Test: atest -v CtsBackupTestCases
Test: atest -v CtsBackupHostTestCases

Change-Id: Ib94c168f8e89b0ba8f398152ea744fe3d626efc4