Rely on Tethering mutex for TetherInterfaceSM

( cherry-pick of 4bc8d6b1 )

Stop passing Tethering's mutex into TetherInterfaceSM, and instead
simply rely on Tethering to acquire its own mutex when accessing
instances of TetherInterfaceSM.

While here, remove some boolean fields (previously guarded by the mutex)
which duplicate state information.

Change-Id: Ie6b2128970981786a5d64fbf700c9f3d0a6721fb
Test: Compiles, unittests pass, WiFi tethering continues to work.
Bug: 28910007