Tablet tweaks: Make notifications bottom-aligned when bottom status bar is enabled

Finally got this to work. If someone can suggest what to do with
compact carrier (it looks ugly alone at the top), I'll implement it
in another patch set (till then I've made it hidden when bottom bar
is enabled)

Screenshot (without compact carrier "fix"): http://puu.sh/29GI

Change-Id: Ic2c857170787fb05d19d8985d6a0a830856fb283