Working around incorrect wallpaper offsets being calculated in RTL.

- When launcher starts up, onCreate() triggers the launcher model loader
  to start, which calls bindScreens() to add the workspace pages.  
  However, layout does not happen until the device is unlocked, which 
  means that even though the default screen index and children are there
  the page scrolls are calculated incorrectly, and even in RTL, the 
  page scroll for the 0th screen is zero (it should be at the right
  most edge of the workspace).  This CL works around this by deferring 
  until the first layout after bindScreens() to unlock the wallpaper
  offset from its default bounds.  The workaround is only applied when 
  the launcher activity is first created.

Bug: 28795125
Change-Id: I33da0d7f934f5337d26e69f068f579a32897a837