Android API minimum for the Swift SDK for Android

[grynspan (Jonathan Grynspan)]

@marcprux

I'll see what happens when I try building for API 21. Anyone have an even lower Android API they need to target?

@Finagolfin

I don't think API 21 is worth it, as they plan to drop it in the coming year, making 23 the lowest available in the NDK.

My two cents: if Google and/or the OHA only supports API level n and up, we should follow suit.