Fix a couple bugs with back button visibility

- Fix back button flickering when returning to NORMAL state instead of
  finishing a gesture to, say, ALL_APPS. This is achieved by posting
  changes to the back visibility on the UI thread (binder calls are
  posted on the background thread).
- Restore back button when touch service reconnects (in case we were
  forced into an app when launcher is force-stopped, for instance).

Change-Id: I3731ee43d66965ce7cd8cf026174aabfe2d5c56c