ENGR00216809 Add null pointer check before do addStorage()

Before /mnt/sdcard primary storage mounted, the MtpStorage
instance is not created, and this cause AndroidRuntime
crash when calling the storage's method with PTP enabled on boot.
So we must add a sanity check here, do not add storage for
null one, until the storage is mounted, added by volume listener.

Signed-off-by: Xinyu Chen <xinyu.chen@freescale.com>
ENGR00217015 Music: Thumbnail of each song show Gallery's picture

MediaProvider will try to fall back to the image in the same folder
of music, if MetadataRetriver failed to extract the AlbumArt image.
MediaProvider will bypass the albumArt image search if the music located
in the root external storage. For those mp3 file in the root dir of /extsd,
need to add the extsd and udisk dir checking

Signed-off-by: guoyin.chen <guoyin.chen@freescale.com>