Apply spring forces to animate to the final position for swipe home

Now instead of an incorrect hack that simulated accelerating to the target,
we actually apply spring forces to make it feel realistic and work no matter
where the target is.

Added two helper classes for this:
- FlingSpringAnim handles the fling, applying friction until reaching the target,
  then a spring to pull towards the final position (also applies if fling wasn't
  in the right direction or strong enough to reach the target).
- RectFSpringAnim uses 2 FlingSpringAnims (x + y) to animate from a starting rect
  to a target rect. It also has an animation to scale from the start rect to the
  target rect, sending progress update callbacks to the caller.

Bug: 123900446
Change-Id: Iafa89db1d55c42816acfa9f1bb84a7519b69ff12

src/com/android/launcher3/anim/FlingSpringAnim.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.launcher3.anim;
+
+import androidx.dynamicanimation.animation.DynamicAnimation.OnAnimationEndListener;
+import androidx.dynamicanimation.animation.FlingAnimation;
+import androidx.dynamicanimation.animation.FloatPropertyCompat;
+import androidx.dynamicanimation.animation.SpringAnimation;
+import androidx.dynamicanimation.animation.SpringForce;
+
+/**
+ * Given a property to animate and a target value and starting velocity, first apply friction to
+ * the fling until we pass the target, then apply a spring force to pull towards the target.
+ */
+public class FlingSpringAnim {
+
+    private static final float FLING_FRICTION = 1.5f;
+    // Have the spring pull towards the target if we've slowed down too much before reaching it.
+    private static final float FLING_END_THRESHOLD_PX = 50f;
+    private static final float SPRING_STIFFNESS = 350f;
+    private static final float SPRING_DAMPING = SpringForce.DAMPING_RATIO_LOW_BOUNCY;
+
+    private final FlingAnimation mFlingAnim;
+
+    public <K> FlingSpringAnim(K object, FloatPropertyCompat<K> property, float startPosition,
+            float targetPosition, float startVelocity, OnAnimationEndListener onEndListener) {
+        mFlingAnim = new FlingAnimation(object, property)
+                .setFriction(FLING_FRICTION)
+                .setMinimumVisibleChange(FLING_END_THRESHOLD_PX)
+                .setStartVelocity(startVelocity)
+                .setMinValue(Math.min(startPosition, targetPosition))
+                .setMaxValue(Math.max(startPosition, targetPosition));
+        mFlingAnim.addEndListener(((animation, canceled, value, velocity) -> {
+            SpringAnimation springAnim = new SpringAnimation(object, property)
+                    .setStartVelocity(velocity)
+                    .setSpring(new SpringForce(targetPosition)
+                            .setStiffness(SPRING_STIFFNESS)
+                            .setDampingRatio(SPRING_DAMPING));
+            springAnim.addEndListener(onEndListener);
+            springAnim.start();
+        }));
+    }
+
+    public void start() {
+        mFlingAnim.start();
+    }
+}