Gallery2: Remove slideshow option if all files of the folder are video

Enter Gallery and open folder whose files are all video, press
menu key should no slideshow option.

Add condition to remove slideshow if the folder are all video
files.

CRs-Fixed: 574984

Change-Id: Ib1603f1609b6490e44039ab64abefb44f14bd477
Merge "Gallery2: Remove slideshow option if all files of the folder are video" into du44