Add template version to AndroidManifest.xml and Info.plist

[filiph]

For tracking purposes, we're adding a key-value pair to each game template. This can be used by the AdMob plugin to recognize that this is a Flutter game built on top of the template.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[domesticmouse]

Is template version all you want to track? Also, you may want to add a comment justifying to the template user why they should leave it in their published game.

[filiph]

I've changed the wording.