Rename IMMS#mCurrentUserId to #mCurrentImeUserId

This is a follow up CL to my previous CL [1], which replaced

  InputMethodManagerService#mSettings

with

  InputMethodManagerService#mCurrentUserId.

As "current user ID" can easily be interpreted as the user ID from

  ActivityManagerInternal#getCurrentUserId(),

explicitly calling it "mCurrentImeUserId" may make more sense in
InputMethodManagerService, where "current user" has always meant the
current IME user since per-profile IME was introduced [2].

This is a mechanical renaming CL. There must be no user-observable
behavior change.

See also Bug 350386877 on our on-going effort on cleaning up the
remaining dependencies on the "current IME user" concept, which is no
longer used in concurrent multi-user IME mode.

 [1]: Iaa7f08c9081aac9abe0c95d522185632d575d6b8
      dc1fd9b2
 [2]: I854ce92b2bf3aab49f14f6cde444acf2182b9ad0
      30026c85

This is a mechanical renaming CL. There must be no observable behavior
change.

Fix: 357187520
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Iba7e59c0d6bed5646737ad89a6825dd6f3c9f082