Reorganize drawable for large and xlarge device (DO NOT MERGE)

Because the holo theme drawables have "holo" effect around 9-patch as
fixed size image, so we need dedicated drawables for each sw600dp-mdpi
and sw768dp-mdpi resolutions for visual designers.

 * drawable-mdpi         (fallback resources)
 * drawable-sw600dp-mdpi (7" tablet, phone style keyboard, placeholder for now)
 * drawable-sw768dp-mdpi (10" tablet, tablet style keybaord)

Note that:
 * Eventually sw600dp drawables will be updated by designers.
 * We need sw768dp drawable copies because, without these, 10" tablet
   will use updated sw600dp drawables wrongly.
 * We have to keep drawable-mdpi as final fallback resources.

Bug: 4436327
Change-Id: I93db87ee99c2abb8d18e869e5411e3cb5bec1020