According to this report: http://www.idc.com/getdoc.jsp?containerId=prUS25450615

In 2014 Android had an 81.5% market share compared to 14.8% for iOS.

So by being iOS only, you've given up on any revenue from 81.5% of the market.

So, to your question "Is it still necessary to be on Android?" - the answer is yes.