Fix broken unit test.

- Looks like recent changes have made the setDataSource() followed by
  prepare() on a MediaPlayer that has an illegal data source start
  throwing an IllegalStateException in the prepare() method rather than
  an IOException in the setDataSource().
- This change just says that if anything goes wrong with the background
  prepare work, rather than specifically an IOException, then we will
  show the user that we failed to play.

Bug: 5266872
Change-Id: Idb6e7ea13f24996abb5da93d2979d3d0bdbf5cbb