Implementation of setOemNetworkPreference

Main implementation of ConnectivityService.setOemNetworkPreference. This
covers the main requirements of this method including listener
functionality.

Bug: 176495594
Bug: 177101287
Bug: 176494815
Test: atest FrameworksNetTests
atest NetworkStackTests
atest FrameworksNetIntegrationTests
atest NetworkStackIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: I8d318ab07785e52dd84d6261fdea8f318dce9bc5