Add header protection for Taskbar All Apps search bar.

Works similar to ActivityAllAppsContainerView<->ScrimView
implementation, except utilizes AbstractSlideInView's builtin scrim.

Test: Manual
Flag: ENABLE_ALL_APPS_SEARCH_IN_TASKBAR=false
Bug: 216683257
Change-Id: Ide41ac47ecffc232fffac34c6bfdda67c1da549d

src/com/android/launcher3/views/AbstractSlideInView.java

@@ -99,7 +99,7 @@ public abstract class AbstractSlideInView<T extends Context & ActivityContext>
     protected @Nullable OnCloseListener mOnCloseBeginListener;
     protected List<OnCloseListener> mOnCloseListeners = new ArrayList<>();
 
-    private final AnimatedFloat mSlideInViewScale =
+    protected final AnimatedFloat mSlideInViewScale =
             new AnimatedFloat(this::onScaleProgressChanged, VIEW_NO_SCALE);
     protected boolean mIsBackProgressing;
     @Nullable private Drawable mContentBackground;