Update target and support libs to SDK 21, fixing compile issues

OpenKeychain/src/main/res/layout/actionbar_custom_view_done.xml

@@ -19,7 +19,6 @@
     android:layout_height="match_parent"
     android:dividerPadding="12dp"
     android:orientation="horizontal"
-    android:divider="@drawable/abc_list_divider_holo_light"
     android:showDividers="end" >
 
     <include layout="@layout/actionbar_include_done_button" />

OpenKeychain/src/main/res/layout/actionbar_custom_view_done_cancel.xml

@@ -18,7 +18,6 @@
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:dividerPadding="12dp"
-    android:divider="@drawable/abc_list_divider_holo_light"
     android:orientation="horizontal"
     android:showDividers="middle">