Clean up class members in Tethering.TetherInterfaceSM

( cherry-pick of 7b30e548 )

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.