adb: fix old host transport selection.

We regressed handling of the old host transport selection syntax, which
broke users that reimplement adb themselves (e.g. Studio via ddmlib).

Bug: https://issuetracker.google.com/140369526
Test: adb raw "host-serial:822X0028S:forward:tcp:42929;localabstract:/com.example.ndktest-0/platform-1568299082100.sock"
Test: ./test_device.py
Change-Id: Iaaec8fde952316fe9bf2a6f6c6c4a3bc9f74bf72