Remove the hardcoded color in main page

Since we don't have grey background in settings page, we could
delete selectable_card and use default selectableItemBackground
instead.

Fix: 28129641
Fix: 36508801
Test: Screenshot
Change-Id: I2b61d8f2214df681a0db7f697fcc6a87dc7d78cb
diff --git a/res/drawable/selectable_card.xml b/res/drawable/selectable_card.xml
deleted file mode 100644
index 3a1e8b2..0000000
--- a/res/drawable/selectable_card.xml
+++ /dev/null
@@ -1,22 +0,0 @@
-<?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.
--->
-
-<ripple
-    xmlns:android="http://schemas.android.com/apk/res/android"
-    android:color="?android:attr/colorControlHighlight">
-    <item android:drawable="?android:attr/colorBackground"/>
-</ripple>
\ No newline at end of file
diff --git a/res/layout/dashboard_tile.xml b/res/layout/dashboard_tile.xml
index 0e06d18..1ec1730 100644
--- a/res/layout/dashboard_tile.xml
+++ b/res/layout/dashboard_tile.xml
@@ -19,7 +19,7 @@
     android:id="@+id/dashboard_tile"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
-    android:background="@drawable/selectable_card"
+    android:background="?android:attr/selectableItemBackground"
     android:gravity="center_vertical"
     android:minHeight="@dimen/dashboard_tile_minimum_height"
     android:clickable="true"
diff --git a/res/layout/settings_main_dashboard.xml b/res/layout/settings_main_dashboard.xml
index 1b7506e..c10193c 100644
--- a/res/layout/settings_main_dashboard.xml
+++ b/res/layout/settings_main_dashboard.xml
@@ -21,5 +21,4 @@
              android:id="@+id/main_content"
              android:layout_height="match_parent"
              android:layout_width="match_parent"
-             android:background="@color/material_grey_300"
              />
diff --git a/res/layout/support_tile.xml b/res/layout/support_tile.xml
index e294868..c2081c7 100644
--- a/res/layout/support_tile.xml
+++ b/res/layout/support_tile.xml
@@ -19,7 +19,7 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
-    android:background="@drawable/selectable_card"
+    android:background="?android:attr/selectableItemBackground"
     android:gravity="center_vertical"
     android:minHeight="@dimen/support_tile_min_height"
     android:orientation="horizontal">