SystemUI: Set Status bar opaque during drawer pulldown on tablets only

The status bar only needs to be set as opaque when the notification
drawer is pulled down on 600dp and 720dp devices. On phones, this
is unnecessary as the top header of the drawer covers the status
bar on the phone UI. This change makes a huge difference in
reponsiveness on devices that run the phone UI as there is no
longer a delay before the drawer appears when interacting with it
on an activity with transclucent window decor.

Patch Set 3: Fix formatting, adjust comment
Patch Set 5: Delete file derp

Change-Id: I932bb92a5245f0323d3317776efbdc9151db449d
SystemUI: fix calculating alpha for back button in landscape mode.

Change-Id: Iea7a2eed86d4b65d3772b95f613d1853c66a0158