Introduce IMMI#getEnabledInputMethodListAsUser()

This is a preparation before fixing Bug 354333072.

Apparently

  InputMethodManagerInternal#getEnabledInputMethodListAsUser()

should have been there for

  KeyboardLayoutManager#getImeInfoListForLayoutMapping(),

which needs to query each user's IME information from the
system_server [1].

This is an internal refactoring. There must be no behavior change
until the above new internal method starts being used.

 [1]: I9057e793641950e2ad903404d591b4e517e91e38
      c2e31eb4

Bug: 354333072
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I78292896d75e5e54e76ed5595f79d917e0a3818a