mobile/test: Have the iOS and Android integration tests send requests to a locally spun up Envoy instead of external websites

A lot of our Android and iOS mobile integration tests send requests to an external website like example.com.
We shouldn't do this - it makes for non-hermetic tests and is not good for the external website to get such traffic.

For most of the integration tests, we can create a local Envoy test server, but for some of them that create an Envoy test server for other purposes (xDS, proxy support), we will have to change the test infra b/c the Envoy test server can only be a singleton right now