Fix WifiDialog app showing a solid background on U-on-T

Fix WifiDialog app showing a solid background on U-on-T with the
following changes:
1) Manually declare a transparent theme for WifiDialog.
2) Don't override getResources() with ServiceWifiResources and
   instead reference ServiceWifiResources directly when needed.
3) Use public device default dialog themes instead of the private
   Theme.DeviceDefault.Dialog.Alert.DayNight.
4) Manually set the textColor of the P2P PIN input since it's not
   visible anymore with new style settings.

Bug: 238353074
Test: manually verify dialog looks correct using `adb shell cmd wifi
launch-dialog-simple` and `adb shell cmd wifi
launch-dialog-p2p-invitation-received` on U-on-T.
Updated-Overlayable: Not applicable - changing default value

Change-Id: If47ba975ffc8706e6457b7ed6fc4f7e3bb23986b
6 files changed