[Multi-user] Rename GlobalBackupManagerService -> BackupManagerService

Restore original naming since UserBackupManagerService is now merged.

Just find and replace, no functionality changes in this CL.

Bug: 118520567
Test: 1) atest RunFrameworksServicesRoboTests
2) atest $(find \
frameworks/base/services/tests/servicestests/src/com/android/server/backup \
-name '*Test.java')
3) adb shell bmgr flows
Change-Id: Ia37997cec93fac9ebb5102edfff9000c9cff4eb1