Rename the user-to-execute-as to "targetUser".

Aligns it with more common conventions of "target" meaning the
potentially different user we'll be executing as. And keeping "calling"
to refer to the original user who made the request.

Bug: 194413082
Test: presubmit
Change-Id: Id762ed7be8da783f7372841b68e22f088a889b97
1 file changed