MediaScanner: Don't delete entries of secondary storage on shutdown

MediaScanner deletes the entries of secondary storage whenever it
is ejected by the user. However, a shutdown also sends this eject
events. This patch ignores the eject event if the system is being
shutdown.

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I0dfe4bd750c563e99b59ad6e9155b241c3228e0d