Handle PhoneStatusBarPolicy user switch callbacks on main thread

Said callbacks poke around StatusBarIconController, which lives on
main thread and is not thread safe.

BUG=27047911
Bug:28613935

Change-Id: I880d79a237b03c06d72b5dff3db24bd60c7b8839
(cherry picked from commit 4f65092f5c95b89f48a48a916467b78d2a07ff89)
1 file changed