adb: extract atransport's connection interface.

As step one of refactoring atransport to separate out protocol handling
from its underlying connection, extract atransport's existing
hand-rolled connection vtable out to its own abstract interface.

This should not change behavior except in one case: emulators are
now treated as TCP devices for the purposes of `adb disconnect`.

Test: python test_device.py, with walleye over USB + TCP
Test: manually connecting and disconnecting devices/emulators
Change-Id: I877b8027e567cc6a7461749432b49f6cb2c2f0d7