Update comments about locking in TrafficController

Add some documentation to explain the usage of mMutex inside
TrafficController.

Bug: 130334320
Test: build
Change-Id: I5e8d8703c4e5b0bba6460aac4fdeba822270a268
Merged-In: I5e8d8703c4e5b0bba6460aac4fdeba822270a268
(cherry picked from commit 2fda2ed286060fa8fe10e3d61d36ba28ef376ce0)
1 file changed