Slice of Provider Model: add slice structure

Implement a slice which can show/update carrier networks.
Before and after: go/b173971144screenshot

Bug: 173971144
Test: atest NetworkProviderWorkerTest  (PASS)
atest ProviderModelSliceTest  (PASS)

Change-Id: I3f0dab364c88723ef3185a2ff040b1fbd1b099f4

src/com/android/settings/wifi/slice/WifiScanWorker.java

@@ -61,7 +61,7 @@ public class WifiScanWorker extends SliceBackgroundWorker<WifiSliceItem> impleme
     @VisibleForTesting
     final LifecycleRegistry mLifecycleRegistry;
     @VisibleForTesting
-    WifiPickerTracker mWifiPickerTracker;
+    protected WifiPickerTracker mWifiPickerTracker;
     // Worker thread used for WifiPickerTracker work
     private final HandlerThread mWorkerThread;