Themes: Add helper methods for creating complete component maps

These methods can be used to apply a theme while replacing any
components not found in the theme with the defaults.

Default components come from the default theme for the system
with any missing components in that default theme coming from HOLO.

Change-Id: Idea55cc684ccc8304b01a991c36b6993f55b8880
Themes: Add STYLE_THUMBNAIL to PreviewColumns

Change-Id: Ie18264cbede92220335400428e67735866cd442d