Retrieve AdSize again after addListener with a getter function getBannerSize()

Hi there, I am new to this plugin and I have a feature request.

**Is your feature request related to a problem? Please describe.**
I cannot find how to get the height of a banner ad. I see the `addListener`options, but after these are fired and my state changes, I do not see an option to get the size of the ad again. Now I am (very ugly) saving the state in localStorage.

**Describe the solution you'd like**
I could like to see an option to say `getBannerSize() -> BannerAdPluginEvents.BannerAdSize`

**Describe alternatives you've considered**
I am now saving the size in localStorage.

Start at the banner ad API and the addListener entry point, then trace how BannerAdPluginEvents.BannerAdSize is exposed after events fire. Confirm the expected getter behavior and platform scope before implementing it, and consider how existing tests cover banner size retrieval; done means callers can retrieve the current banner size without persisting it themselves.

Written by the indexing model from the issue text.