MediaScanner: delete abnormal elements in prescan procedure.

There is a race in MediaProvider's mUnmountReciver.
when it doing external storage's database clean up at power down procedure,
if the datebase is big, system may shutdown after db.update, but db.delete
not done, there will be many elements with no filename in the database.
That may cause duplicated files and folders shown in MTP.

This patch clean up these abnormal elements in MediaScanner's prescan procedure.

Change-Id: I0a3dacffd12892b1a2a892b2749a02ab3dea109b
Author: Liang Wang <liangx.wang@intel.com>
Signed-off-by: Liang Wang <liangx.wang@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 58969