android doc browser: avoid explicit background color without a foreground one

E.g. on my device the foreground color was white, the explicit light
background color made the filenames almost unreadable.

Change-Id: I64e361447ef9af4f3998afeb4a938907b6ca7eb3

android/experimental/LOAndroid3/res/layout/file_grid.xml

@@ -19,7 +19,6 @@
         android:verticalSpacing="10dp"
         android:horizontalSpacing="10dp"
         android:stretchMode="columnWidth"
-        android:background="#ffeeeeee"
         android:gravity="center">
     </GridView>