Have WebView Implementation Dev Setting use DefaultAppPickerFragment.

Make the WebView Implementation Dev Setting look like the fullscreen
settings deriving from DefaultAppPickerFragment.
Point the Activity opened through Settings.ACTION_WEBVIEW_SETTINGS to
the class WebViewAppPicker which is our new implementation of the
WebView Implementation setting.

Ensure the new setting closes if it is reached from a user that isn't an
admin of the device.

Bug: 34806477
Bug: 34966439
Test: Ensure WebView implementation Dev Setting looks ok (disabled
packages should have a text showing why they cannot be chosen).
Test: Start Intent with action Settings.ACTION_WEBVIEW_SETTINGS and
ensure the started Activity looks similar to the WebView Implementation
Dev Setting.
Test: Ensure picking a package that is no longer active, updates the
setting (so that package isn't visible anymore).
Change-Id: I08007c515193739ad61dfd735bb5130fc07bd6e6

src/com/android/settings/core/gateway/SettingsGateway.java

@@ -121,6 +121,7 @@ import com.android.settings.system.SystemDashboardFragment;
 import com.android.settings.tts.TextToSpeechSettings;
 import com.android.settings.users.UserSettings;
 import com.android.settings.vpn2.VpnSettings;
+import com.android.settings.webview.WebViewAppPicker;
 import com.android.settings.wfd.WifiDisplaySettings;
 import com.android.settings.wifi.ConfigureWifiSettings;
 import com.android.settings.wifi.SavedAccessPointsWifiSettings;
@@ -248,6 +249,7 @@ public class SettingsGateway {
             AppAndNotificationDashboardFragment.class.getName(),
             UserAndAccountDashboardFragment.class.getName(),
             EnterprisePrivacySettings.class.getName(),
+            WebViewAppPicker.class.getName(),
     };
 
     public static final String[] SETTINGS_FOR_RESTRICTED = {