Use UserManagerInternal when possible in IMMS

This is a preparation to support per-profile IME in inline-reply.

For some operations implemented in UserManagerService,
UserManagerInternal would be better choice when called from
InputMethodManagerService because it becomes clear that security check
is required to use those internal APIs.

There should be no visible behavior change in this CL.

Bug: 120744418
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I5b80c2225a26c8f67047b84d7e90e53c47b8567b