Start MountService before performBootDexOpt

This change is to start Mountservice before starting
performBootDexOpt, as in one case, in performBootDexOpt
when system upgrade happens, StorageManager will be started to
get the low threshold of DataDir. But, at this point, as
Mountservice is still not up, StorageManager will end up
having a null object of Mountservice.

Change-Id: If2b5e1b58e7d2a72c6313f196e98a68738295ec6