Merge pull request #8408 from gtorodelvalle/dialer-bug-816941-overlays-transition-follow-up

Follow up to bug 816941 - Solving the opacity issue (r=etiennesegonzac)(cherry picked from commit b80b3763b32d753030894eee2294d414d0fd4177)
Merge pull request #8492 from gtorodelvalle/dialer-bug-816941-overlays-transition-follow-up-2

Bug 847866 - [Dialer] Avoid race conditions in overlays (r=etiennesegonzac)(cherry picked from commit 3444dd7ff1467e7b09e4c0bc3185a46f0a20b188)