adb: don't abort when connecting to the same address twice.

When connecting to an address, we construct a transport first, and then
check whether we've already connected to that address. The consequent
destruction of the BlockingConnectionAdapter attempts to join threads
that haven't been started, which aborts.

Make it safe to destruct a BlockingConnectionAdapter without calling
Start on it first, to solve this.

Bug: http://b/69137547
Test: nc -l 12345 & (adb connect localhost:12345; adb connect localhost:12345)
Test: python test_adb.py
Change-Id: I6cb968a62dbac6332907e06575893d764905ee62