Bug 830644 - [Music][Hidpi]Support high resolution devices

* Added @2x assets (we are waiting to get the new album covers at @2x)
* Added background-size declarations

In order to test this PR you need to:
 * **HIDPI Device:** `HIDPI=1 make install-gaia` 
 * **non-HIDPI Device:** `make install-gaia` 
 * **B2G desktop:** run it with `--screen=480x800` flag

Please @dominickuo take a look