Update rewarded ad loading/bailout flow

[mhkawano]

This introduces three related changes, and are bundled as one so that I can test them together.

1. We will not show anything until we've confirmed that a rewarded ad can be shown, effectively removing the loading view.
2. We will not block a rewarded ad user if they cannot show the prompt, and do not have an alternative action.
3. We will pass locked to the article endpoint based on the page config.

b/328640563
b/328639440
b/328640557