Enable IS_INTERNAL_BUILD for sample app
Enable IS_INTERNAL_BUILD for sample app

Summary:
This sets the build flavor to "internalBuild" which enables the
IS_INTERNAL_BUILD flag.

This is meant to be a temporary workaround to enable Stetho when building the
sample app from the repo.

Reviewed By: IanChilds

Differential Revision: D4907649

fbshipit-source-id: 777e5f436e3ac69f64729755bcaae2519f794156

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho