Add getThemeRes to WizardManagerHelper

Add helper method to get the base theme defined by setup wizard. This
is intended for use by clients who doesn't require extra theme
attributes in their activities (i.e. use @style/SuwThemeGlif directly
rather than extending it).

Test: ./gradlew test
Change-Id: I22872d918c346bd7adffe4be909d73fae23f44dc