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
diff --git a/media/extractors/mp4/ItemTable.h b/media/extractors/mp4/ItemTable.h
index 5a6af5e..6591271 100644
--- a/media/extractors/mp4/ItemTable.h
+++ b/media/extractors/mp4/ItemTable.h
@@ -76,8 +76,8 @@
     std::set<uint32_t> mBoxesSeen;
 
     bool mImageItemsValid;
-    uint32_t mCurrentImageIndex;
-    KeyedVector<uint32_t, ImageItem> mItemIdToImageMap;
+    uint32_t mCurrentItemIndex;
+    KeyedVector<uint32_t, ImageItem> mItemIdToItemMap;
 
     status_t parseIlocBox(off64_t offset, size_t size);
     status_t parseIinfBox(off64_t offset, size_t size);