Cleanup how we set back button alpha

Add BackButtonAlphaHandler to set back button alpha, instead of setting
it from multiple places.

Also force back button alpha to be 1 if swipe up is disabled (b/80091187)

Change-Id: I49b63a0e6b033a3a947a847669a398f1b9ff0564

quickstep/src/com/android/launcher3/uioverrides/BackButtonAlphaHandler.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2018 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.uioverrides;
+
+import android.animation.ValueAnimator;
+
+import com.android.launcher3.Launcher;
+import com.android.launcher3.LauncherState;
+import com.android.launcher3.LauncherStateManager;
+import com.android.launcher3.anim.AnimatorSetBuilder;
+import com.android.quickstep.OverviewInteractionState;
+
+public class BackButtonAlphaHandler implements LauncherStateManager.StateHandler {
+
+    private static final String TAG = "BackButtonAlphaHandler";
+
+    private final Launcher mLauncher;
+    private final OverviewInteractionState mOverviewInteractionState;
+
+    public BackButtonAlphaHandler(Launcher launcher) {
+        mLauncher = launcher;
+        mOverviewInteractionState = OverviewInteractionState.getInstance(mLauncher);
+    }
+
+    @Override
+    public void setState(LauncherState state) {
+        UiFactory.onLauncherStateOrFocusChanged(mLauncher);
+    }
+
+    @Override
+    public void setStateWithAnimation(LauncherState toState,
+            AnimatorSetBuilder builder, LauncherStateManager.AnimationConfig config) {
+        if (!config.playNonAtomicComponent()) {
+            return;
+        }
+        float fromAlpha = mOverviewInteractionState.getBackButtonAlpha();
+        float toAlpha = toState.hideBackButton ? 0 : 1;
+        if (Float.compare(fromAlpha, toAlpha) != 0) {
+            ValueAnimator anim = ValueAnimator.ofFloat(fromAlpha, toAlpha);
+            anim.setDuration(config.duration);
+            anim.addUpdateListener(valueAnimator -> {
+                final float alpha = (float) valueAnimator.getAnimatedValue();
+                mOverviewInteractionState.setBackButtonAlpha(alpha, false);
+            });
+            builder.play(anim);
+        }
+    }
+}