Reserve item ids for samples + grid

Reserve item ids for samples + grid, if the file size is small
than tile size, the mNumTiles is 1. The numItemsToReserve will
equal to mNumTiles, not reserve id for grid item. It will
cause grid item been over written, can not find the file size.

Change the numItemsToReserve calculate expression from mNumTiles > 1
to mNumTiles > 0, to reserve id space for grid item.

Bug: 162797911
Change-Id: I18638f1e9026282044c7df73f8a2d78d58a13447