docs: Fixed logic for determining image sample size

The code sample in the "Load a Scaled Down Version into Memory"
section on the "Loading Large Bitmaps Efficiently" page now handles
corner cases correctly when determining how to downscale an image by
a factor that is equal to a power of 2.

Bug: 25944661
Change-Id: I971f90fb5d153abec4bc1d96ae465e3910e82efa