The two apps you mentioned are open source. You could just grab them, build them and load them. Here are links to some of the code you'll need: https://android.googlesource.com/platform/packages/apps/Emai... https://android.googlesource.com/platform/packages/apps/Cale... https://android.googlesource.com/platform/packages/apps/Exch...

I've never built them outside of a whole platform build so you might (read: probably) will hit issues. This could be a fun project for someone and it sounds like there is demand.