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
Change-Id: I45a98795c571ac97bd6025988da156bd06d57e45