add dark assets to sw900dp devices.

Bug: 34078632
Test: cts-tradefed run commandAndExit cts-dev \
  --module CtsSystemUiTestCases \
  --test android.systemui.cts.LightBarTests \
  --disable-reboot --skip-device-info \
  --skip-all-system-status-check \
  --skip-preconditions
Change-Id: I72b5d67be7a25f3726cee66d6e35db7242ca265a