Don't use attr for color value in drawables.

According to the following this is not supported:

 * http://stackoverflow.com/a/13471695/2391921

This uses the approach in that SO answer, by extracting the attribute to
instead point at a drawable, and have one drawable for each theme.

Fixes #979.

app/src/main/res/drawable/category_preview_app_card_background_dark.xml

@@ -5,5 +5,5 @@
 -->
 <shape xmlns:android="http://schemas.android.com/apk/res/android">
     <corners android:radius="4dp" />
-    <solid android:color="?attr/categoryPreviewAppCardBackground" />
+    <solid android:color="#212121" />
 </shape>

app/src/main/res/drawable/category_preview_app_card_background_light.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Visually different from other app cards because it doesn't have a drop shadow, and has a larger
+  corner radius.
+-->
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <corners android:radius="4dp" />
+    <solid android:color="#faf8ef" />
+</shape>

app/src/main/res/drawable/details_panel_donate_background_dark.xml

@@ -2,5 +2,5 @@
 <shape xmlns:android="http://schemas.android.com/apk/res/android"
     android:shape="rectangle">
     <corners android:radius="3dp" />
-    <solid android:color="?attr/detailPanel" />
+    <solid android:color="#303030" />
 </shape>

app/src/main/res/drawable/details_panel_donate_background_light.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+    <corners android:radius="3dp" />
+    <solid android:color="#eff4f9" />
+</shape>

app/src/main/res/layout/app_card_normal.xml

@@ -4,7 +4,7 @@
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="100dp"
     android:layout_height="130dp"
-    android:background="@drawable/category_preview_app_card_background"
+    android:background="?attr/categoryPreviewAppCardBackground"
     android:padding="8dp">
 
     <!-- Ignore ContentDescription because it is kind of meaningless to have TTS read out "App icon"

app/src/main/res/layout/app_details2_donate.xml

@@ -7,7 +7,7 @@
     android:layout_margin="@dimen/details_activity_padding"
     android:clickable="true"
     android:orientation="vertical"
-    android:background="@drawable/details_panel_light_background"
+    android:background="?attr/detailPanel"
     android:padding="@dimen/details_activity_padding">
 
     <TextView

app/src/main/res/values/attrs.xml

@@ -3,12 +3,12 @@
     <declare-styleable name="Theme">
         <attr name="appDetailsBackground" format="color" />
         <attr name="appDetailsCardBackground" format="color" />
-        <attr name="categoryPreviewAppCardBackground" format="color" />
+        <attr name="categoryPreviewAppCardBackground" format="reference" />
         <attr name="mainTabSwapBackground" format="color" />
         <attr name="mainTabSwapSplashTint" format="color" />
         <attr name="categoryName" format="color" />
         <attr name="installedApps" format="color" />
-        <attr name="detailPanel" format="color" />
+        <attr name="detailPanel" format="reference" />
         <attr name="appListItem" format="color" />
     </declare-styleable>
 </resources>

app/src/main/res/values/styles.xml

@@ -14,12 +14,12 @@
         <item name="actionBarPopupTheme">@style/AppThemeLight.Toolbar</item>
         <item name="appDetailsBackground">#0c0c0c</item>
         <item name="appDetailsCardBackground">#000000</item>
-        <item name="categoryPreviewAppCardBackground">#303030</item>
+        <item name="categoryPreviewAppCardBackground">@drawable/category_preview_app_card_background_dark</item>
         <item name="mainTabSwapBackground">#0a0a0a</item>
         <item name="mainTabSwapSplashTint">#050505</item>
         <item name="categoryName">#ffffff</item>
         <item name="installedApps">#ffffff</item>
-        <item name="detailPanel">#212121</item>
+        <item name="detailPanel">@drawable/details_panel_donate_background_dark</item>
         <item name="appListItem">#ffffff</item>
     </style>
 
@@ -36,12 +36,12 @@
         <item name="actionBarPopupTheme">@style/AppThemeLight.Toolbar</item>
         <item name="appDetailsBackground">#fcfcfc</item>
         <item name="appDetailsCardBackground">#ffffff</item>
-        <item name="categoryPreviewAppCardBackground">#faf8ef</item>
+        <item name="categoryPreviewAppCardBackground">@drawable/category_preview_app_card_background_light</item>
         <item name="mainTabSwapBackground">#fafafa</item>
         <item name="mainTabSwapSplashTint">#f5f5f5</item>
         <item name="categoryName">#4a4a4a</item>
         <item name="installedApps">#424242</item>
-        <item name="detailPanel">#eff4f9</item>
+        <item name="detailPanel">@drawable/details_panel_donate_background_light</item>
         <item name="appListItem">#424242</item>
     </style>