Move Wu check for max colors earlier.

This has no impact. However, after going through the quantizers line by
line for other bug fixes, after getting a break from looking at them
in-depth for a couple weeks, it was much more readable/felt safer to
adjust mMaxColors at the top.

Bug: 182333325
Test: Verify output remains constant,
atest CtsAppTestCases:android.app.cts.WallpaperColorsTest#fromDrawableTest
passes.

Change-Id: I561ba44bca392634a956992fd946f3310aa1c8ee