[Multi-user] Consolidate BMS creation

Part of prep work to create a UserBMS instance per user.

Moves BMS state initialization to the BMS constructor. Most of this
state will then be moved to the UserBMS constructor since it's per-user
state.

Bug: 120212806
Test: 1) atest RunFrameworksServicesRoboTests
2) atest TrampolineTest
3) boot -> unlock user -> create service
4) adb shell bmgr flows

Change-Id: I140c4db3d5c147e3dd09e590eaf15a9f4d8b3da1