Tweak UI for action buttons under app header.

- Make a transparent button with border style

Bug: 1584976
Test: manual
Change-Id: Ide53d71963afd4587834357999b40df81bb93500

res/drawable/button_bg_transparent.xml

@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2016 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.
+-->
+
+<inset xmlns:android="http://schemas.android.com/apk/res/android"
+       android:insetLeft="4dp"
+       android:insetTop="6dp"
+       android:insetRight="4dp"
+       android:insetBottom="6dp">
+    <ripple android:color="?android:attr/colorControlHighlight">
+        <item android:id="@android:id/mask">
+            <shape android:shape="rectangle">
+                <corners android:radius="2dp"/>
+                <solid android:color="@android:color/white"/>
+            </shape>
+        </item>
+        <item>
+            <shape android:shape="rectangle">
+                <solid android:color="#00ffffff"/>
+                <!--make button border solid color, nontransparent-->
+                <stroke android:color="?android:attr/colorAccent" android:width="1dp"/>
+                <corners android:radius="2dp"/>
+            </shape>
+        </item>
+    </ripple>
+</inset>