Retrieve WifiDppQrGeneratorFragment at runtime.

* Add getWifiDppQrCodeGeneratorFragment() in WifiFeatureProvider
* WifiDppConfiguratorActivity calls from the FeatureFactory to get WifiDppQrCodeGeneratorFragment instance
* Use a flag to guard this behaviour
* Updated a few private variables of WifiDppQrCodeGeneratorFragment to
  protected so I can access them in derived classes.

Bug: b/338891273
Settings Intake: b/329012096

Test: manual test, override the flag locally, and check I can switch to
overriden implementation.
Flag: com.android.settings.flags.enable_wifi_sharing_runtime_fragment

Change-Id: I715bf89bd9be221f06914e87629c76bf7a2f743f