Make InputMethodBindingController per user

Make InputMethodBindingController per user by moving
IMMS#mBindingController to `UserDataRepository.UserData`.

This is the final roll-forward of [1], which was reverted by [2]
due to an additional locking contention in IMMS constructor [3].

[1] Original CL: I870a76ac1d196436f1b2f172d65ac46d580650d6
[2] Revert   CL: I31bf05f1d5af17b0220a4ea0df9278a1155beacf
[3] Sync IMMS Constructor CL: I8e51b1ced4dc16cdca7e898885c64793665fafef

This CL represents an internal refactoring and shouldn't introduce any
observable breakage.

Bug: 325515685
Test: atest FrameworksInputMethodSystemServerTests
Test: atest CtsInputMethodTestCases
Test: atest FrameworksServicesTests
Test: atest --host FrameworksInputMethodSystemServerTestsRavenwood

Change-Id: Id6e9eca6caaa8edde8c4a405448c31c0ca986509