Fix fling gestures when expanding the AppBar from RecyclerView

Fling gestures don't work when expanding the AppBar. Define a new
behavior class to fix them

https://issuetracker.google.com/issues/37058076
https://stackoverflow.com/a/32454407

proguard.flags

@@ -1,2 +1,4 @@
 -keep class android.support.design.widget.** { *; }
 -keep interface android.support.design.widget.** { *; }
+
+-keep class org.lineageos.updater.ui.FlingBehavior { *; }

res/layout/activity_updates.xml

@@ -11,7 +11,8 @@
         android:id="@+id/app_bar"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:fitsSystemWindows="true">
+        android:fitsSystemWindows="true"
+        app:layout_behavior="org.lineageos.updater.ui.FlingBehavior">
 
         <android.support.design.widget.CollapsingToolbarLayout
             android:id="@+id/collapsing_toolbar"

src/org/lineageos/updater/ui/FlingBehavior.java

@@ -0,0 +1,43 @@
+package org.lineageos.updater.ui;
+
+import android.content.Context;
+import android.support.design.widget.AppBarLayout;
+import android.support.design.widget.CoordinatorLayout;
+import android.support.v7.widget.RecyclerView;
+import android.util.AttributeSet;
+import android.view.View;
+
+public final class FlingBehavior extends AppBarLayout.Behavior {
+    private static final int TOP_CHILD_FLING_THRESHOLD = 3;
+    private boolean isPositive;
+
+    public FlingBehavior() {
+    }
+
+    public FlingBehavior(Context context, AttributeSet attrs) {
+        super(context, attrs);
+    }
+
+    @Override
+    public boolean onNestedFling(CoordinatorLayout coordinatorLayout, AppBarLayout child,
+            View target, float velocityX, float velocityY, boolean consumed) {
+        if (velocityY > 0 && !isPositive || velocityY < 0 && isPositive) {
+            velocityY = velocityY * -1;
+        }
+        if (target instanceof RecyclerView && velocityY < 0) {
+            final RecyclerView recyclerView = (RecyclerView) target;
+            final View firstChild = recyclerView.getChildAt(0);
+            final int childAdapterPosition = recyclerView.getChildAdapterPosition(firstChild);
+            consumed = childAdapterPosition > TOP_CHILD_FLING_THRESHOLD;
+        }
+        return super.onNestedFling(coordinatorLayout, child, target, velocityX, velocityY,
+                consumed);
+    }
+
+    @Override
+    public void onNestedPreScroll(CoordinatorLayout coordinatorLayout, AppBarLayout child,
+            View target, int dx, int dy, int[] consumed) {
+        super.onNestedPreScroll(coordinatorLayout, child, target, dx, dy, consumed);
+        isPositive = dy > 0;
+    }
+}