Simplify IMMS constructor a bit

This is a preparation before making

  InputMethodManagerService#mImeDrawsImeNavBarRes

multi-user ready.

This CL aims to simplify the the constructor of

  InputMethodManagerService

without changing any observable behavior by extracting out test-only
dependency setup.

Bug: 345519864
Test: presubmit
Test: atest FrameworksInputMethodSystemServerTests
Flag: EXEMPT refactor
Change-Id: Id8aec502d60231644c9c8a9f45373de71df9ddc6