[MS07.1] Move NetworkStatsCollection/IdentitySet into frameworks

These files are needed for the data migration util system Api
interfaces to allow OEMs to construct NetworkStats* objects.
Thus, they need to be moved into android.net package, and some
of them will be exposed as @SystemApi in T.

Eventually these classes will be moved into the Connectivity
module, but in the mean time they will be temporarily moved to
f/b/package/ConnectivityT for the preparation stage.
However, the tests are already in the module. Therefore for
the S-derived branch, the test cannot see the renamed classes
since any framework CLs will not be auto-merged into this branch.
Thus, the tests need to stay disabled on the S-derived branch,
and will be re-enabled after all files are moved into the module.

Test: TH
Bug: 197717846

Change-Id: I95278a99cf2437ada28001161ceea17a1d32f2a4