Use DisplayMetrics directly for thumbnail sizes.

An eagle-eyed reviewer noted that going through density means we'd
adjust based on user display scale settings, which we don't need to
do for photos, since they'll always be rendered at the native display
density.

Bug: 63440459
Test: manual
Change-Id: Iad7e891b8c70cdf23e14e95873d62b6af41b0fcd
1 file changed