Fix crash when number of pixels < max colors

If extraction is run on a set of pixels whose count is less than the
number of max colors, then colors.length < max colors, and accesses
to colors[index] inside the for loop throw an ArrayOutOfBoundsException.

Test: add widget to home screen, reduce size of widget to small area.
observe crash. `mp core services` with fix, observe that crash no longer
occurs. `mp core services` again without fix, verify crash occurs after
boot.
Fixes: 181130209

Change-Id: I60bf1ab39f7a69f2fb0745049fd46d2f0b53a12c