[Multi-user] Clean up BMS structure

Part of preparation to extract out user BMS.

Remove unused components:
- Backup traces
- Unused fields

Clean-up work:
- Better organize BMS class structure to be able to separate global vs.
user state. This is purely a move and no functionality has changed:
> static fields, static methods, instance fields, constructor, methods,
private static methods, static inner class (system service definition).
- Add javadoc comments to comply with new checkstyle preupload hook.

Bug: 118520567
Test: 1) atest RunFrameworksServicesRoboTests
2) atest FrameworksServicesTests

Change-Id: I5ac868ff0df0ec007b64d686647d9a676e374e40