Fix [a11y] Wi-Fi Easy Connect Talkback issues

1. Remove android:contentDescription from ButtonPreference layout file
   'wifi_button_preference_widget.xml' because it should be a more
   general purpose component. Add ButtonPreference#setButtonContentDescription
   for this change.
2. Add a LinearLayout to group title & summary for better Talkback UX
3. Set android:contentDescription for Wi-Fi enrollee scan button
4. setTitle for Talkback actionbar back button of WifiDppQrCodeGeneratorFragment &
   WifiDppQrCodeScannerFragment
5. Auto trigger Talkback to speak title & summary in WifiDppAddDeviceFragment &
   WifiDppChooseSavedWifiNetworkFragment
6. Auto trigger Talkback to speak summary change in WifiDppAddDeviceFragment
7. Auto trigger Talkback to speak error message in WifiDppQrCodeScannerFragment

Bug: 126007405
Bug: 124424996
Bug: 124424445
Test: manual test

Change-Id: I54a3f033bb8871c47be12115ae8f97691fd83302

src/com/android/settings/wifi/dpp/WifiDppChooseSavedWifiNetworkFragment.java

@@ -23,6 +23,7 @@ import android.os.Bundle;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
+import android.view.accessibility.AccessibilityEvent;
 import android.widget.Button;
 import android.widget.ListView;
 
@@ -105,5 +106,11 @@ public class WifiDppChooseSavedWifiNetworkFragment extends WifiDppQrCodeBaseFrag
 
         mButtonRight = view.findViewById(R.id.button_right);
         mButtonRight.setVisibility(View.GONE);
+
+        if (savedInstanceState == null) {
+            // For Talkback to describe this fragment
+            mTitleSummaryContainer.sendAccessibilityEvent(
+                    AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED);
+        }
     }
 }