Add previews to the themes contract.

Change-Id: I607a6eda1c63a701516d2bc78ca6b705841fbdf7
Remove DEFAULT_THEME_APPLIED_ON_FIRST_BOOT

doApplyDefaultTheme should be able to apply the default theme, if
one exists, whenever applyDefaultTheme() is called.

Change-Id: Ia50b4089d122855a4ddde91a02eef33ed3f9eaf4

Conflicts:
	core/java/android/provider/Settings.java