Changes To Storage Muxer to Increase Ease-of-Use
Changes To Storage Muxer to Increase Ease-of-Use

While working with integrating storage muxer into the codebase, there
were some painful points in the API. This change addresses those
painful points.

1. Getting cells - many times we know that the cell should be under
                   a specific path. So resolving the path and
                   checking for null got annoying. This adds
                   a call that assume it should find the cell.

2. Get active cell - when getting the active cell, we often need
                     the cell and the path to the cell. So this
                     creates a "Handle" which pairs the cell and
                     path together.

3. Adjusting registry - when testing we need to change what storage
                        cells are actually available. Instead of
                        unregistering and registering cells, this
                        adds methods to add and remove a temporary
                        override.

Issue #1248

Change-Id: Ifd45ee5912f53b1da444476b560cf03669a19b11

tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,TheModMaker/shaka-player,TheModMaker/shaka-player