Deprecate NetworkPolicyCallback and related APIs

The callback is no longer used by any client modules since Android VIC.
The definition is kept to maintain source compatibility with older
versions.

Flag: android.net.platform.flags.deprecate_network_policy_callback

Test: N/A

Bug: 353342610
Change-Id: I3242924c032e9fe9738f0b03c34e2ce4054293ef