Fix crash on density / font change in NotificationGuts.gutsContent

There were two issues:

1) The mGutsMenuItem wasn’t getting nulled out correctly
2) There was no check for guts != null when seeing if there were
   exposed guts, this meant the bind would happen when there were no
   guts exposed, because mGutsMenuItem wasn’t nulled out correctly, it
   would be out of date / still attached to a parent.

Test: Manually tried:
1) Have notifications that *haven’t* had guts revealed
2) Change screen density

1) Have notifications, reveal some of their guts
2) Change screen density

Bug: 34737513
Change-Id: Iff0a782c9377aec973eb07645e852bc320dbb101