Support minSdk 21

[ashughes]

I see you changed minSdk support from 26 to 22 in e53a414, which is great. Is there any particular reason this wasn't set to 21? There's not much difference between 21 (Android Lollipop 5.0) and 22 (Android Lollipop 5.1), so I assume there wouldn't need to be any additional work to make this change. Our app supports down to minSdk 21, so we'd this to adopt Superwall.

[ianrumac]

Hey @ashughes!
Thanks for the suggestion - there was no specific reason for not setting it to 21, but as it's older than 10 years and it's usage is below 0.1% worldwide, we did not consider it. We'll have to test it out first, but if everything goes right we'll reduce the version to 21!