Added option to disable xip
Added option to disable xip

Summary:
When running an app on a real iPhone in debug mode, I occasionally get issues when trying to load the js bundle and I suspect the issue is with xip.io (and it doesn't look like I'm the only one: https://github.com/facebook/react-native/issues/12786 https://github.com/facebook/react-native/issues/9688#issuecomment-272591839). So I've added the ability to optionally disable the use of xip.io if an env variable `DISABLE_XIP` is set.

Add `export DISABLE_XIP=true` to the `Bundle React Native code and images` build phase. Run the app on a real iPhone and ensure that it can load the JS bundle from the host computers IP.
Closes https://github.com/facebook/react-native/pull/13326

Differential Revision: D4855719

Pulled By: ericvicenti

fbshipit-source-id: cb2e91291acadaa78ea302800b55c2e5388f6380

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