Change dark theme screen to toggle

The dark theme screen has been removed in favor of a simple
toggle with a dialog that shows up the first time a user
tries to enable dark theme when in light theme.

Test: robotests pass
Bug: 130251804
Change-Id: I48f8e24a2b2a117e5a8054c5bc0b240ba68fe1ad

res/drawable/dark_theme.xml

@@ -0,0 +1,26 @@
+<?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
+  -->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+        android:width="48dp"
+        android:height="48dp"
+        android:viewportWidth="24.0"
+        android:viewportHeight="24.0"
+        android:tint="?android:attr/colorAccent">
+    <path
+        android:fillColor="#FF000000"
+        android:pathData="M12,22C17.52,22 22,17.52 22,12 22,6.48 17.52,2 12,2 6.48,2 2,6.48 2,12 2,17.52 6.48,22 12,22ZM12,3.915c3.889,0 8,4.005 8,8.085 0,4.08 -3.927,7.992 -7.928,7.992z"/>
+</vector>