Start aw-sync optionally

[ishanarora]

Thank you for your amazing work!

aw-qt v0.12.3b15 onwards has the option to enable aw-sync module. It would be great to add the option to aw-android beta.

I expect this would also involve:

• [ ] adding a preference button in the onboarding activity to pick the sync directory
• [ ] implement hostname
• [ ] enable android's buckets in aw-sync