Marking setOemNetworkPreference as @SystemApi

Marking setOemNetworkPreference in ConnectivityManager as @SystemApi.

Bug: 176496438
Bug: 176494815
Test: atest FrameworksNetTests
atest FrameworksNetIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: I4681c88dc3a83f71c387b29610c33594e57cb43f
5 files changed