Avoid dividing 0 by 0 in StackScrollAlgorithm.

This is very likely the cause of b/233692725 (which is essentially an
error saying that translationZ is set to Float.NaN). Looked through all
the codepaths modifying translationZ, and this was the only one I could
find that could generate NaN.

Test: Not quite sure how to test it though, since I can't reproduce.
Will add some logging in a follow-up commit.
Bug: 233692725

Change-Id: Ie5db92ee714b2265ceacc366144bdadad00aaf51

packages/SystemUI/src/com/android/systemui/statusbar/notification/stack/StackScrollAlgorithm.java

@@ -884,6 +884,9 @@ public class StackScrollAlgorithm {
                 childViewState.zTranslation = baseZ;
             } else {
                 float factor = (notificationEnd - shelfStart) / shelfHeight;
+                if (Float.isNaN(factor)) { // Avoid problems when the above is 0/0.
+                    factor = 1.0f;
+                }
                 factor = Math.min(factor, 1.0f);
                 childViewState.zTranslation = baseZ + factor * zDistanceBetweenElements;
             }