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
ef85d880a5