Move FakeSettingsProvider to a common location.

(cherry picked from commit 7df1a828)

Test: runtest frameworks-util -c com.android.internal.util.test.FakeSettingsProviderTest
Test: runtest frameworks-services -c com.android.server.ConnectivityServiceTest
Test: runtest frameworks-services -c com.android.server.retaildemo.PreloadAppsInstallerTest
Test: runtest frameworks-services -c com.android.server.retaildemo.RetailDemoModeServiceTest
Bug: 31479480
Change-Id: I8d4d4de2937012dd33924f53e68f96241a9b2b4f