adb: move list-forward, kill-forward back into handle_forward_request.

The daemon-side reverse functions depended on handle_forward_request:
move them back instead of duplicating the logic we had in
handle_host_request. Accomplish what we originally wanted to do in this
change by changing the transport argument of handle_forward_request to a
std::function that acquires a transport, either via
acquire_one_transport or immediately returning a value that we already
have.

As a side effect, fix a bug where we would emit spurious errors for host
service requests.

Bug: http://b/112009742
Test: echo "001chost:connect:127.0.0.1:5555" | nc localhost 5037
Test: python test_device.py
Test: python test_adb.py
Change-Id: Iccc555575df6dbd7de10382854c4ea2c6f4beeaa