Fix Icon size in DocumentsUI

For XXHDPI devices if you go into Settings/Display and Notification light
and select a custom value (application) you will see a GIANT icon.

Before - http://i.imgur.com/DRkua3T.png

After - http://i.imgur.com/f7xW4tY.png

Change-Id: Ia355bddc515906134c8b3b4e67d381aaacceec78