Cleans up bad state when transient taskbar shows on home.

Bug: 279514548
Test: open transparent activity on top of launcher
      unstash taskbar
      go home
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:3bad3ebaad529a8be587b9c0eae61faf87ea8505)
Merged-In: I13ab79b334e1f8feda441a82cc4d035c0142f513
Change-Id: I13ab79b334e1f8feda441a82cc4d035c0142f513