Stop relying on IActivityManager#getCurrentUser() in IMMS

There is no good reason for InputMethodManagerService to continue using

  IActivityManager#getCurrentUser()

to get the current user ID because such information is already
available through

  ActivityManagerInternal#getCurrentUserId().

There should be no observable behavior change in this CL.

Bug: 234882948
Test: presubmit
Change-Id: Id386400df4d45c0b710902e60bb0715c6e1fc3bb