int userHandle -> int userId

Throughout the codebase, sometimes an integer user ID is called
userId, and sometimes it is called userHandle. Here, we rename the
occurences of "int userHandle" to "int userId" in select files.
Additionally, we add the @UserIdInt annotation in many more places.

This certainly does not cover all uses of userHandle, which is used
widely throughout Android; doing so would be more dangerous and cause
merge conflicts. But at least the core User code can have a consistent
nomenclature.

Bug: 138384792
Test: compiles. presubmit passes.

Change-Id: I9a2684e9bfac637e0476c4b68027008ebb8a160b