Add api level to resource

[breedx-splk]

The semantic conventions include an Android specific attribute called android.os.api_level:

https://github.com/open-telemetry/semantic-conventions/blob/c98600cf91251effcb4eba4129711ecf45c3d4bf/docs/resource/android.md

This attribute is "recommended" so we should include it in the AndroidResource.

[marandaneto]

I think it's not needed for v1 since this can be added later, it is not a behavior or API surface breakage, unless doing this requires changes in the API surface