Block the user from enabling or disabling webview fallback packages.

Now that we support webview fallback packages - packages that should be
enabled if and only if no other webview packages are available - we need
to ensure that the Settings UI consistently shows that these packages
cannot be enabled or disabled (e.g. the 'Enable' and 'Disable' buttons
for enabling/disabling them are greyed out).

Also, remove the Dialog that lets a user enable a disabled webview
package from the webview implementation Dev Setting. Instead show a
Toast if the user has chosen an invalid package.

Bug: 26375524, 26375860

Change-Id: I949083d3f7c83cd2e049dd2c5c15ec5ab880fe07

src/com/android/settings/applications/InstalledAppDetails.java

@@ -61,6 +61,7 @@ import android.view.MenuInflater;
 import android.view.MenuItem;
 import android.view.View;
 import android.view.ViewGroup;
+import android.webkit.IWebViewUpdateService;
 import android.widget.Button;
 import android.widget.ImageView;
 import android.widget.TextView;
@@ -254,6 +255,16 @@ public class InstalledAppDetails extends AppInfoBase
             enabled = false;
         }
 
+        try {
+            IWebViewUpdateService webviewUpdateService =
+                IWebViewUpdateService.Stub.asInterface(ServiceManager.getService("webviewupdate"));
+            if (webviewUpdateService.isFallbackPackage(mAppEntry.info.packageName)) {
+                enabled = false;
+            }
+        } catch (RemoteException e) {
+            throw new RuntimeException(e);
+        }
+
         mUninstallButton.setEnabled(enabled);
         if (enabled) {
             // Register listener