adbd: properly handle FUNCTIONFS_BIND without ENABLE.

If a device is unplugged, a BIND will happen without an ENABLE until the
device is plugged in, which would previously result in adbd crashing
every 5 seconds due to an attempt to join a thread that hadn't been
started.

Bug: http://b/128255355
Test: manually observed adbd via serial cable while unplugged
Change-Id: I90d6384e7e829df1ffaf73a95f2418a2ff97fb03