adb: fix deadlock between transport_unref and usb_close.

Fix a deadlock that happened when a reader/writer thread released a
transport while the hotplug thread attempted  to handle a device
disconnection. Decrementing a transport refcount to zero would hold the
global transport mutex and attempt to take the usb handles mutex, while
the hotplug thread would hold the usb handles mutex and try to call
unregister_usb_transport, which would attempt to take the global
transport mutex.

Resolve this by making transport_unref not take the global transport
mutex.

Bug: http://b/62423753
Test: python test_device.py
Change-Id: Ib48b80a2091a254527f3a7d945b6a11fae61f937