Fixed lint warnings

app/lint.xml

@@ -0,0 +1,8 @@
+<lint>
+    <issue id="UnusedResources">
+        <ignore path="res/drawable"/>
+        <ignore path="res/layout"/>
+        <ignore path="res/menu"/>
+        <ignore path="res/values"/>
+    </issue>
+</lint>

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

@@ -4,7 +4,7 @@
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:layout_gravity="center"
-    android:background="?android:attr/selectableItemBackground"
+    android:foreground="?android:attr/selectableItemBackground"
     android:clickable="true"
     android:focusable="true"
     android:orientation="vertical"

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

@@ -4,7 +4,7 @@
     android:id="@+id/book_item_container"
     android:layout_width="match_parent"
     android:layout_height="72dp"
-    android:background="?android:attr/selectableItemBackground">
+    android:foreground="?android:attr/selectableItemBackground">
 
     <ImageView
         android:id="@+id/book_image"