[1/2] Themes: Multiwallpaper support

1. Add wallpaper component id to ThemeChangeRequest

2. Query themes provider for the wallpaper path given a pkgName and wallpaper id.
   Previously the path was only assumed from the pkgName.

3. Update the mix'n'match table to reflect the component id

Change-Id: Ia38da1c36b3c2f7f038d0a34054aff2257a294dc