Rename InputMethodSettings#mCurrentUserId to #mUserId

Given that we are going to keep maintaining one InputMethodSettings
instance for each user regardless of whether the user is foreground or
background, having "CurrentUser" in InputMethodSettings' field/method
names is just misleading.

Let's drop "Current" from the following field and method names.

 * InputMethodSettings#mCurrentUserId
 * InputMethodSettings#getCurrentUserId()

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

Bug: 309837937
Test: presubmit
Change-Id: I546e1dabb39845aa16322e4e32cc016074280b7c