Do not show static overlays as a theme

Static overlays aren't changeable by a user. So we shouldn't list them
as a theme.

This CL fixed b/36812704 as well to run unit tests.

Test: building succeeded and tested on sailfish.
      make RunSettingsRoboTests
      adb shell am instrument -w -e class \
          com.android.settings.display.ThemePreferenceControllerTest \
          com.android.settings.tests.unit/android.support.test.runner.AndroidJUnitRunner
Bug: 37480890
Bug: 36812704

Change-Id: I8c13b6956083095dceaab3da6642bef1647d6a30