More Theme Test Fixes

Bug 5404694

This fixes ThemeTest for HDPI devices but not the ActivitySnapshot or
SplitActivitySnapshot tests.

- Fix the Android.mk that specified 32 dpi instead of 320 dpi
- Add some Bitmap#recycle() calls to avoid OOMs and remove showing
  the generated and reference images in the app. They weren't showing
  up and it goes too fast to be useful imho...
- Add updated HDPI assets
- Avoid building all of CTS in themetestscripts.sh

Change-Id: I6f08099afe0f7d949c3c149da89ffea1cd0812fc