Improve app buffering, error states

The app's UI currently assumes that once you start a stream, the stream will commence fairly soon. Since we don't control a customers connection, and don't host the podcast content this isn't a guaranteed. We need to add some user interface indicators of buffering progress and more information about any errors or slow downs around that. This issue will require some design input.

Related iOS issue: https://github.com/Automattic/pocket-casts-ios/issues/8

Start by reviewing the issue requirements and the related iOS issue at https://github.com/Automattic/pocket-casts-ios/issues/8. The work needs design input to define buffering progress, slowdowns, and error information; it is done when those states are specified and implemented in the Android app.

Written by the indexing model from the issue text.