Add an option allowing to skip wallpaper apps selection.

This commit adds an option to the Settings application, making it
possible to launch a different activity for wallpaper selection

Also removing te intent constant android.settings.WALLPAPER_SETTINGS
The target for this action can't be easily overriden when the
wallpaper target is overriden, which would provide an inconsistant
experience.

Bug: 28554244
Bug: 26688904
Change-Id: I25367fbb4505615351fbad9e1aa1ca88df0cd034