Store the effective UID in NetworkRequestInfo.

This is necessary for privileged components such as
JobScheduler to file NetworkCallbacks on behalf of other
UIDs.

This CL adds the field to NetworkRequestInfo and updates some
of the matching code, but does not set the field to anything
other than the UID that created the request. Thus, it should
cause no behaviour changes.

Bug: 165835257
Test: refactoring with no change in behaviour, passes existing tests
Change-Id: I21a22ed1b851b8511d36f5202c9eac03e7158d3d