[google_adsense] Add optional init parameters.

[ditman]

Adds AdSenseCodeParameters configuration object for adSense.initialize.

Fixes a typo in the MATCHED_CONTENT_ROWS_NUM and MATCHED_CONTENT_COLUMNS_NUM constants, that would have resulted in passing the wrong parameter name to the AdSense JS.

This PR also continues the tightening of exports by making all the exports from barrel files explicit, so it's harder to accidentally expose any unintended API surface.

Issues

• Continuation of: [google_adsense] Tighten exports and docs. #8233
• Part of: Support Google AdSense in Flutter Web applications flutter#40376

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