Refactor Publisher app to allow ad url to be set in initializeFromStaticData

[c4mb0t]

When this change to PrivacySandboxViewModel.kt was made in the Publisher app, it resulted breaking the rendering of the winning ad during the ad selection flow.

Initial investigation shows that this happens because initializeFromStaticData is called after the call to retrieveAd() in init, which results in the winning ad url being overwritten with an empty string.

We should try to figure out why this happens, and what is the best way to refactor the app so that the logic flow is more intuitive.