[CS15]Do not use hidden API of PlatformProperties

ConnectivityService is going to become a mainline module, and
it will not able to use hidden method anymore. Using PlatformProperties
as a static library instead of hidden API.

Bug: 170917042
Test: atest FrameworksNetTests
Merged-In: I3a3deca5d2e0f690db8c0061de2db2217376d268
Change-Id: I3a3deca5d2e0f690db8c0061de2db2217376d268