Move the png open and destroy functions into a class

The open_png() function used to open the png file but didn't close it;
and this caused the leak of fd. However, we cannot close the file inside
open_png() because the png file needs to remain open until the outer
function finishes parsing the file and destroys the png struct.

This CL addresses this issue by implementing a PngReader class to handle
the creation/destruction of the png struct.

Bug: 67010912
Test: Run graphic tests; also run locale tests and check fd.
Change-Id: I9a803b3cd8c16f16a9ffe8f0acc7fe0f42e95eb0