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