Premium Content: Correct "non-static method call" errors.

[kwight]

The Premium Content block causes log errors by calling a non-static method statically to get tokens. This PR corrects the code usage to avoid the errors.

Discovered by @blackjackkent while testing #44192 .

Testing Instructions

• Set up a JN test site that is connected to WP.com and has a JP Premium plan attached.
• Sandbox the store and API. Set the JN site to use the sandboxed API (WP Admin > Settings > Jetpack Constants).
• Install the latest Gutenberg plugin.
• Use the build artifact from the FSE plugin / Build FSE plugin (pull_request) task below as a ZIP plugin that you can upload to the test site.
• Create a post, and add a Premium Content block for testing.
• Save the post, and open the front-end of the post in an incognito window.
• Subscribe to the content, using the test store card credentials to purchase.
• The page will reload to show the now-accessible content; Verify it loads without the above errors.

[matticbot]

Test live: https://calypso.live/?branch=fix/pc-jp-warnings

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

Caution: This PR affects files in the FSE Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D46534-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing in the FSE Plugin for more info: PCYsg-ly5-p2

[kwight]

This still leaves the "header information" issues:

[gwwar]

Thanks @kwight I verified that this ties the non-static method call, and that the premium content block still behaves as expected when subscribing.

[gwwar]

Going to help land to help clear up the team board.