Replace HttpUrlConnection backend with OkHttp

[lberrymage]

Using HttpUrlConnection for HTTP downloads has various drawbacks, most of them stemming from the fact that new features in the downloader take much more manual effort and code to implement. On the other hand, OkHttp supports many features we want out-of-the-box and makes implementation much simpler for the rest, including the following:

• Support asynchronous APK downloads #95
• Offline support #150
• Cache app-specific repository metadata #159
• Implement ETag checks to avoid unnecessary repodata fetches #8
• Implement download resumption for interrupted downloads #10