MountService: Only notify PackageManager for true ASEC changes

MountService can be dealing with multiple mounts (emmc, sdcard), but
only the EXTERNAL_STORAGE has any ASEC mounts on it.  Since the PMS
updateExternalMediaStatus doesn't specify the path, we need to make sure
it's only called for the real ASEC mount.  Otherwise we get into nasty
race conditions when multiple mounts are coming and going, like during
UMS transitions, and sdcard-installed apps tend to break.

Change-Id: I85c6a601e84afd30b44270b0892686c2d864ce8d