Add 1.5x dimens for screens that are large enough

For phones with smallest width at least 480 dp, we can scale
everything by 1.5x to fill in the space better. For phones with less
width than this, we maintain the current 320 dp specification.

Bug: 131610834
Test: Emulate on 240x320 ldpi, 480x800 hdpi, 480x800 mdpi, 800x1280 mdpi
on both portrait and landscape and layouts work as intended
Change-Id: I3f7d28dc22aa3a2de96b3de51b1aa5b5c2cb19fc