Improve theme application transition.

- Remove duplicate app info changed / activity relaunch messages caused by
applying several overlay packages sequentially; this eliminated the
double launcher activity restart
- Asynchronously enable each overlay from SystemUI; this increases the
speed of enabling overlays, thus decreasing the effect of the frozen
activity transition

Fixes: 130151718
Test: make; change themes many times
Change-Id: Idcf04edc88cb9391b9920b9e1f9f873a5b62b311