Fixing header jump

Linking header position to an empty entry in the recyclerView,
instead of calculating the vertical scroll position. This
allows the header to be in sync with the recyclerView scroll and
item animations

Other simplifications:
> Moving top collapse handle out of header view (it doesn't scroll)
> Removing background clipping logic from full-sheet
> Moving tab bar inside the header view

Bug: 196464142
Test: Verified on device
Change-Id: Iae5a0ae9af7ce258e1b391b8e85c5c270fe56197

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

@@ -28,6 +28,7 @@ import android.util.AttributeSet;
 import android.util.Property;
 import android.view.MotionEvent;
 import android.view.View;
+import android.view.ViewGroup;
 import android.view.animation.Interpolator;
 
 import com.android.launcher3.AbstractFloatingView;
@@ -68,7 +69,7 @@ public abstract class AbstractSlideInView<T extends Context & ActivityContext>
     protected final SingleAxisSwipeDetector mSwipeDetector;
     protected final ObjectAnimator mOpenCloseAnimator;
 
-    protected View mContent;
+    protected ViewGroup mContent;
     protected final View mColorScrim;
     protected Interpolator mScrollInterpolator;