adb: convert Connection to a nonblocking interface.

Rename the existing Connection to BlockingConnection, add a nonblocking
Connection, and add an adapter between the two, to enable future work
to reduce the impedance mismatch from implementing a blocking interface
on top of nonblocking primitives.

While we're here, delete A_SYNC, and remove one layer of pipes when
sending a packet (replacing it with a condition variable when using
BlockingConnectionAdapter).

Test: python test_device.py, manually plugging/unplugging devices
Change-Id: Ieac2bf937471d9d494075575f07e53b589aba20a