Move 3rd party wallpaper pickers to the front

This is a feature request from Steve so here you go!

Change-Id: Iac6a26c7b51752d65026a871712c3590caa926d9
Include default wallpaper in LockscreenWallpaperPickerActivity

Change-Id: I0e3d46688bdb257bc32e10d9255dfc4fbf7beb4c
Add navbar background preview to ThemesContract

Change-Id: Id3ed43c86649b1757fec7f3fc7dee2d2730258e2