Fix up PackageManager tests

PackageManagerTests was trying to mount and unmount the media without
waiting for the correct state to arrive. It was proceeding with any
state transition.

Some tests had their own volume mounting and unmounting code that was
flawed, so switch it to the more robust mountMedia() and unmountMedia()
implementations.

Change-Id: If464927bb09dd15cb040c6cce195a6ad8452de95