Remove @hide usage and fix NetworkPolicy backup

Remove usage of hidden symbols for network stats APIs, replacing them
with public API alternatives.

Also fix backup of NetworkPolicy when the set of subscriber IDs is
empty. This as not caught because the test used a hidden API builder
that creates a non-empty set containing null.

Bug: 197717846
Test: atest FrameworksCoreTests
  (cherry-picked and fixed merged conflict from ag/16781267)
Change-Id: I45a98795c571ac97bd6025988da156bd06d57e45
Merged-In: I45a98795c571ac97bd6025988da156bd06d57e45