Make getNetworkSpecifier() public API.

The builder lets clients set it, and this may be useful for
unit tests of apps. It should be public.
The need arises from uses of this in WiFi and Telephony
network factories.

Test: build
Bug: 135998869
Change-Id: I57279cac139c28e8654d2066ba0c60edd1e6cd98
2 files changed