DngCreator: Assume square pixel centered at (x+0.5, y+0.5)

Example 1:
active_array_size: 3 x 3, optical center: (1.5, 1.5),
normalized optical center: (0.5, 0.5)

Example 2:
active_array_size: 2 * 2, optical center: (1, 1),
normalized optical center: (0.5, 0.5)

This reverts commit 47849abc.

Test: DngCreatorTest
Bug: 119566614
Change-Id: Id3177d2056bc83a73ee1a32de0b74d51161ab56b