Fix bug in AssetAtlas packing loop

There is a bug in AssetAtlasService that it always choose the
MAX_SIZE as height. The for loop of height calculation starts
from MAX_SIZE and breaks when it finds a working texture. This
means the height loop will always break for the initial value.
Reording the loop optimize the algorithm to find a smaller
atlas texture. The MIN_SIZE is also changed to 512 for fewer
assets case.

Bug: 26429912
Change-Id: Ic0b2396e9697c417a24f81ec792f8a8af53915ad