Fix crash when quantizing

Incoming pixels are negative numbers. Performing this operation
ensures we receive an accurate alpha value, without it, all pixels have negative alphas.

Fixes: 182523380
Test: Add logs, see that alphas _before_ this patch are logged
as negative numbers, causing all pixels to be marked as transparent and ignored in quantization. Add patch, see logs
indicate valid alphas.

Change-Id: Ice7b7db245de9dd089cffc490b2e24780f9c025b

core/java/com/android/internal/graphics/palette/WSMeansQuantizer.java

@@ -75,7 +75,7 @@ public class WSMeansQuantizer implements Quantizer {
             // Note: they don't _have_ to be ignored, for example, we could instead turn them
             // opaque. Traditionally, including outside Android, quantizers ignore transparent
             // pixels, so that strategy was chosen.
-            int alpha = (pixel >> 24);
+            int alpha = (pixel >> 24) & 0xff;
             if (alpha < 255) {
                 continue;
             }