Round display scaling densities down to the nearest 2 DPI

This will prevent scaling artifacts due to odd densities. We will not
make any adjustments to the "normal" density, which will preserve
tvdpi or whatever the device manufacturer needed to use.

Rounding the density down is always safe, since it will not push us
below our 320dp lower bound on minimum effective screen dimension.

Bug: 27225670
Change-Id: Ib084bb3d4fc70a59106ac74df5394c21e9f8c7bd