Add a unit test for the AppOps permission for displaying UI overlays.

The test logic is the same as ExetrnalSourcesSettingsTest but I moved/renamed some methods so both AppOps tests can use them.

Test: make -j32 SettingsUnitTests && adb install -r out/target/product/marlin/data/app/SettingsUnitTests/SettingsUnitTests.apk && adb shell am instrument -w -e class com.android.settings.applications.ExternalSourcesSettingsTest com.android.settings.tests.unit/android.support.test.runner.AndroidJUnitRunner
Test: same as above, but for DrawOverlaySettingsTest
Change-Id: Ifc0f0985a4cb4a82a00839aeb4cd4216c777250e

tests/unit/src/com/android/settings/applications/DrawOverlaySettingsTest.java

@@ -0,0 +1,35 @@
+/*
+ * 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.applications;
+
+import android.app.AppOpsManager;
+import android.provider.Settings;
+import android.support.test.filters.LargeTest;
+import android.support.test.runner.AndroidJUnit4;
+
+import org.junit.runner.RunWith;
+
+@RunWith(AndroidJUnit4.class)
+@LargeTest
+public class DrawOverlaySettingsTest extends AppOpsSettingsTest {
+
+    public DrawOverlaySettingsTest() {
+        super(Settings.ACTION_MANAGE_OVERLAY_PERMISSION, AppOpsManager.OP_SYSTEM_ALERT_WINDOW);
+    }
+
+    // Test cases are in the superclass.
+}