Adds a check and a fatal error when calling dr_app_setup() in the middle of standalone library usage (such as from a custom allocator using DR heap outside of DR-managed windows). This is much nicer than a mysterious crash that happens otherwise.
Tested on a proposed #4640 (closed) solution to eliminate application heap from droption where the droption static initializers use DR heap and thus invoke standalone mode behind the scenes, while later the app tries to run dr_app_setup().
Issue: #4640 (closed)