UX cleanups.

Visually separate directory background from sidebar.
Lower shadows so the "real" effect isn't so prominent.

Hitchhikders:
Consolidate bool values in config.xml files (were in dimens.xml).
Add missing supportsRecents & supportsSearch methods
    to RootInfo.
Rename RecentLoader to RecentsLoader so it doesn't defy
    searching for "Recents" string.
Move Downloads below the divider.

Change-Id: I06c76e26ec6df6541860a1448865c67ed953ed43