Expose NetworkWatchlistManager#getWatchlistConfigHash

Connectivity is becoming a mainline module in S and
ConnectivityService is used this API for getting watch list
configurations. However, Mainline modules are not allowed to use
non-formal APIs. Thus, make this API as module-lib api to support
the usage.

Bug: 172183305
Test: make update-api
Change-Id: I21ce5f158850892926b92cf6347c61a57ac95b72