Refactor TISM add/remove code

This de-duplicates all the TISM adding and teardown code.
It does not yet change the locking or threading semantics.

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - manual tethering of WiFi and USB start normally
Bug: 32163131
Bug: 36216864
Change-Id: I44a57660d23e76a7c13368cb8f60eaeac5830a0a