Add settings for the assist gesture.

Availability of the gesture is gated by a new FeatureProvider.

Test: make -j RunSettingsRoboTests; manual test on
      supported/unsupported configurations.

Change-Id: I3529367a73e33370d5112b91d5144293ffa7fa22

src/com/android/settings/gestures/AssistGestureFeatureProvider.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2017 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.settings.gestures;
+
+import android.content.Context;
+
+/** Feature provider for the assist gesture. */
+public interface AssistGestureFeatureProvider {
+
+    /** Returns true if the assist gesture is supported. */
+    boolean isSupported(Context context);
+
+}