Show a dialog with a slider to set the back gesture's sensitivity

Bug: 131447780
Test: Manual test on device
Test: make RunSettingsRoboTests ROBOTEST_FILTER=RadioButtonPreferenceWithExtraWidgetTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=SystemNavigationGestureSettingsTest

Change-Id: I9fcd1a50c77689118857326de0cf8082e835b491

res/layout/dialog_back_gesture_sensitivity.xml

@@ -0,0 +1,67 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ 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
+  -->
+
+<LinearLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:orientation="vertical"
+    android:padding="12dp">
+
+    <SeekBar
+        android:id="@+id/back_sensitivity_seekbar"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:layout_gravity="center"
+        android:min="0"
+        android:max="3"
+        style="@android:style/Widget.Material.SeekBar.Discrete"
+        android:theme="@*android:style/ThemeOverlay.DeviceDefault.Accent" />
+
+    <LinearLayout
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:orientation="horizontal"
+        android:layout_marginTop="2dp"
+        android:layout_marginBottom="8dp">
+
+        <TextView android:id="@+id/low_tick"
+                  android:layout_width="match_parent"
+                  android:layout_height="wrap_content"
+                  android:layout_weight="1"
+                  android:layout_marginBottom="2dp"
+                  android:layout_marginStart="12dp"
+                  android:gravity="start"
+                  android:text="@string/low_label"
+                  android:textAppearance="?android:attr/textAppearance"
+                  android:singleLine="true"
+                  android:ellipsize="marquee" />
+
+        <TextView android:id="@+id/high_tick"
+                  android:layout_width="match_parent"
+                  android:layout_height="wrap_content"
+                  android:layout_weight="1"
+                  android:layout_marginBottom="2dp"
+                  android:layout_marginEnd="12dp"
+                  android:gravity="end"
+                  android:text="@string/high_label"
+                  android:textAppearance="?android:attr/textAppearance"
+                  android:singleLine="true"
+                  android:ellipsize="marquee" />
+
+    </LinearLayout>
+</LinearLayout>