Split NetworkUtils and NetworkUtilsInternal

NetworkUtils is planned to move to a dedicated JAR for connectivity
classes, while NetworkUtilsInternal would stay in the
frameworks-minus-apex JAR, in the com.android.internal.net package.

Bug: 171540887
Test: m, boots, wifi working
      atest FrameworksNetTests
Change-Id: I3d38d72ad23a4bf84af823c7baeb6fed25c0665f