surfaceflinger: Use landscape resolution for default dpi

When HWC doesn't provide DPI values for a display, we pick a default
DPI based on resolution. The intent was that 1080p and higher displays
would get XHIGH density, and lower resolutions would get TV density.
In KK (and possibly forever) we had a bug that we'd always use TV
density. That was fixed in L, but that fix exposed a pre-existing bug
that we always used the display's height in its native orientation,
rather than in landscape orientation. So an 800x1280 tablet like N7v1
started getting XHIGH density instead of the intended TV density.

Bug: 17461633
Change-Id: Ia57fa49e61f36bdda63ce283ef62c9953297222c