Sharecropping originally refers to a process where a landowner would let people farm their land in exchange for a portion of the yield. It has negative associations with post-slavery exploitation of black farmers as well as poor farmers in general in the 1800s and early 1900s. In this setting, it's being used to compare the practice of hosting and selling your app on someone else's platform, with those same connotations of exploitation.

Basically they are saying to stop putting your app on the app store and letting google take a large % of the app and letting your business be dependent on another business.

In this case the landlord is Google, and you are the sharecropper, giving a share (30%) to google to be allowed to use the play store.

More importantly: a share whose figure is chosen by the owner, not by you.