Metasmask user-agent

What's the recommend way to detect the user is using the Metamask built-in browser ?

I think user-agent is a good way. probably cannonical way

`export const isMetamaskBrowser = /(Metamask)/i.test(navigator.userAgent)`

However looking at the source the it appears you are not currently adding a custom identifier to the user-agent string

https://github.com/MetaMask/metamask-mobile/blob/bcc22b37381fe59200fc560c42c4712b89106309/app/core/AppConstants.js#L35

Start with app/core/AppConstants.js at line 35 and review the referenced user-agent definition. Check how the built-in browser exposes its user agent and whether the proposed identifier can be verified consistently. Done means the browser can be detected through the documented user-agent behavior, with tests or verification covering the resulting string.

Written by the indexing model from the issue text.