adb: fix use after free of atransport.

libadbd_auth might report authentication success for a transport that's
already been destroyed. Fix this by storing a weak pointer to the
atransport that gets cleared upon destruction instead of a raw pointer.

Bug: http://b/144704376
Test: ./test_adb.py
Test: ./test_device.py
Change-Id: Idffe027381e6b2e37f06aa0166e97cafc98eaf3b