Change SQL to improve performance.

To know the number of images, use "COUNT(*)" instead of "_id" in SQL.
Using "_id" gets all result set. and when getCount() is called, CursorWindow is filled.
But using "COUNT(*)" gets just 1by1 data.
So using "COUNT(*)" is more faster than now.
And this SQL is called everytime per a file during a file transfer in MTP
The better way is by using "EXIST".
But if using "EXIST", modify MediaProvider.call() method.
So i applied just a "COUNT(*)"

Change-Id: Ic5882af0fad2304f836b7b7043a4746359391609
Signed-off-by: muzbit.kim <muzbit.kim@lge.com>