BroadcastReceiver refactoring

All registerReceiver() calls now specify to be run on the tethering
master state machine's handler. Eventually, some of the locking might
be removed (requires careful review, later).

Also: slightly refactor StateReceiver for readability

Test: as follows
    - built (bullhead)
    - flashed
    - booted
    - runtest frameworks-net passes
    - vanilla tethering from WiFi to mobile works
Bug: 32163131
Change-Id: I39844f6e1608179ebeb41668a6db8a4e44d30ecf