Update/automate publishing

This is a quick MR just to modify the fastlane config so I can test out publishing the v2.1.0 beta with it. Note that this MR also removes the x86 abi from being built by gradle, since it doesn't currently work and we have just been deleting it manually before publishing.

I'm going to merge quickly so no need to approve @sphalt2 @TaslimOseni, just added you as reviewers so you are aware of the change.