stagefright: renaming some variable in ItemTable

Renamed most "imageIndex" to "itemIndex". This is a
refactoring with no functional change, in preparation
for upcoming changes to support image sequence retrieval,
as the image index will be referring to the index of
displayable images, instead of the index into item table.

bug: 67652588

Change-Id: If7fc51eb90df7e44e635a756f628a6d344155a79