Allow ContextImpl.setTheme(int) to be called after getTheme()

Previously the theme could only be set if it had never been obtained,
which was inconsistent with the documentation for Context.setTheme()
and the behavior of ContextThemeWrapper.

Also prevents double-apply of themes in setTheme().

Bug: 20756250
Bug: 20230451
Change-Id: I2566b3e4546cd1c68db79f10f01a5a1256fd439c