Improve threading, create dedicated thread queue for podcasts

Refactors some threading code and check the thread priorities set. Also, it put podcast remote feed loading (async tasks)
in a separate thread queue in order to un-block other task that might be running at the same time and get stuck behind
podcast loads on a slow connection (e.g. load downloads/playlist tasks).
Switch to isActiveNetworkMetered API offered by Android SDK >= 16

The app used another fast and free network heuristic so far, this commit switches it to the official API for devices running Jelly Bean or later.
Improve episode description web view

Adapt web view settings to show complete episode description and scale images that might be part of it. This also adopts to the different web view versions used by
the various Android version and makes sure to set correct encoding.