Add theme fallback

- Add FallbackThemeWrapper, which behaves like ContextThemeWrapper,
  except that the base context's theme attributes takes precedence
  over the wrapper context's.
- Use the FallbackThemeWrapper in TemplateLayout (SetupWizardLayout
  and GlifLayout), so that inflating them with the wrong theme will
  not crash, but rather look wrong.

This allows SuwLib to add required attributes without risking
crashing clients that for some reason doesn't use the right themes.

Test: ./gradlew connectedAndroidTest
Change-Id: I694ad7279ce733659f6aa5d72e2087a09d3f1e22