Move shared methods to net shared lib

The NetworkUtils.getNetworkPart() method is used by Settings
and wifi. NetworkUtils is inside the incoming connectivity
module. It will become inaccessible outside the module, so
move the shared methods to shared lib and also move the
related helper method.

The corresponding usgae is also updated.

Bug: 172183305
Test: atest FrameworksNetTests
Change-Id: I47410afa27aad61f63759df41be959f323c1d100