[Multi-user] Extract BackupManagerService to UserBackupManagerService

BMS is split into GlobalBMS and UserBMS.
UserBMS:
- Core backup/restore implementations and bookkeeping.
- Will be per user in a future CL (only one instance in this CL to limit
scope of changes).

GlobalBMS:
- System service definition, interaction with Trampoline, user-aware
operations.
- Will be renamed back to BMS in a future CL (so BMS history can follow
UserBMS in this CL).

*** Reviewers, please pay attention to the patchset breakdown to better
understand what is a rename/move vs. new changes ***

* Base -> Patchset 2: BMS splits into GlobalBMS and UserBMS (MOVE ONLY)

Patchset 1: Rename BMS -> UserBMS
Patchset 2: Move parts of UserBMS to GlobalBMS
No new functionality, only moving existing code.

* Patchset 2 -> Patchset 3: New functionality for GlobalBMS

- Switch references from BMS to GlobalBMS in Trampoline.java.
- Change the definition of the system service in SystemServer.java to
GlobalBMS.
- Instantiate one instance of UserBMS in GlobalBMS constructor.
- Add all IBackupManager methods, which for now just directly calls the
corresponding method in UserBMS.

* Patchset 3 -> Patchset 4: Migrate usages of BMS in code (RENAME ONLY)

Find and replace, no new functionality.

* Patchset 4 -> Patchset 5: Migrate usages of BMS in tests (RENAME ONLY)

Find and replace, no new functionality.

* Patchset 5 -> Patchset 6: New tests for GlobalBMS

Add tests for all the new IBackupManager methods added (just tests
straight redirection for now).

* Overall: View Patchset 2 -> [Latest Patchset] for new code.

TODO: Modify art-profile with rename

Bug: 118520567
Test: 1) atest RunFrameworksServicesRoboTests
2) atest $(find
frameworks/base/services/tests/servicestests/src/com/android/server/backup
-name '*Test.java')
3) atest GtsBackupHostTestCases
4) atest CtsBackupTestCases
5) atest CtsBackupHostTestCases
6) atest DeviceOwnerTest
7) 'Backup Now' in Settings
8) Cloud and d2d restore in SUW; deferred restore
9) All 'adb shell bmgr' flows
10) adb backup; adb restore
Change-Id: Ib5a5837375fe950bc7d33a5e31cca16b605541f9