Clean up class members in Tethering.TetherInterfaceSM

Mark final fields as such.
Group mutex protected fields together with the mutex that protects them.
Always access mutex protected fields under the mutex.

Bug: 28798823
Change-Id: I96cdd5e063babb73e9f124107c5576c47801f34b
Test: Compiles, Wifi tethering continues to work on angler.
(cherry picked from commit 7b30e548)