Slide in volume rocker in natural rotation only.

Change I6e49539080407d71ddfa44a9aba25cd7b4a6c0a2 updated the volume
rocker to only slide in/out if the device is in portrait mode (since most
devices have the volume buttons on the side of the screen in portrait
mode). This doesn't hold on devices which are naturally landscape though
(i.e. have the volume buttons on the side in landscape).

Instead of checking landscape, this CL checks for rotation == 0, since
most devices seem to have zero rotation in their natural posture.

Note that this doesn't just override isLandscape; the ringer type
buttons (i.e. the silent/vibrate/ring selector) should still stack
horizontally in landscape mode, so that part of the SysUI still checks
for landscape instead.

Bug: 182993737
Test: Manually tested; animations are difficult to test for.
Change-Id: Ic67e3012b1c32e5475e05ddf247efbb8fa7413ac