Implement DownloadClient using HttpURLConnection

The version of OkHttp used in AOSP doesn't handle dynamic table size
updates [1] properly [2]. Instead of fixing OkHttp or importing a
prebuilt updated version, implement a new download client only using
HttpURLConnection, which seems to work properly.

[1] https://tools.ietf.org/html/rfc7541#section-6.3
[2] https://trac.nginx.org/nginx/ticket/1397

Change-Id: I3eedf7326f2017812c4a12d41f9ea028d255f7a8