fixed crash on api < 21 due to drawable using attrs

OpenKeychain/src/main/res/drawable/cardview_header.xml

@@ -6,6 +6,6 @@
         android:height="1dp"
         android:width="1000dp" />
 
-    <solid android:color="?attr/colorCardViewHeaderDivider" />
+    <solid android:color="#808080" />
 
 </shape>

OpenKeychain/src/main/res/drawable/fab_label_background.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <shape xmlns:android="http://schemas.android.com/apk/res/android">
-    <solid android:color="?attr/colorFabBackground"/>
+    <!-- using an attribute for color will crash on API <= 20. Refer to http://stackoverflow.com/a/13471695/3000919 -->
+    <solid android:color="#b2000000"/>
     <padding
         android:left="16dp"
         android:top="4dp"

OpenKeychain/src/main/res/drawable/section_header_dark.xml

@@ -6,6 +6,6 @@
         android:height="2dp"
         android:width="1000dp" />
 
-    <solid android:color="?attr/colorHeaderText" />
+    <solid android:color="#d0d0d0" />
 
 </shape>

OpenKeychain/src/main/res/drawable/section_header_light.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle" >
+
+    <size
+        android:height="2dp"
+        android:width="1000dp" />
+
+    <solid android:color="#212121" />
+
+</shape>