ConvertFrom-ExistingSubmission adds incorrect tags

[ConvertFrom-ExistingSubmission.ps1](https://github.com/microsoft/StoreBroker/blob/v2/StoreBroker/ConvertFrom-ExistingSubmission.ps1) adds some incorrect tags with capitalization like '150X150' when the API expects this in lowercase '150x150'. We should pass through the tags and double check that they are all still in the valid formats.