Apollo: Use correct preference keys for artwork download

* These settings were not being respected because the preference key was
  wrong.

Change-Id: I2f6733b6de206a76ce1f6899beced36922653b05
Apollo: Avoid crash when attempting to load many items

* Thread pool executor will reject additional tasks when it has
  exhausted it's queue space causing Apollo to crash.
* Load default artwork if executor is too busy.

Change-Id: Ie25182ede0cfbd0ca8595dbb1f8c024a13a09d93
Apollo: Update default album art

* PS 4: Fix some pixels around the edges
* PS 5: Add xxhdpi asset

Change-Id: Id876a095ea54ebfed1ad7dfc41d678e709e37332