adb: don't hardcode ports in test_adb.

If we get unlucky and something else (or ourselves, in another thread)
beats us to listening on our hardcoded ports, we can deadlock.

Bug: http://b//149829737
Test: ./test_adb.py
Change-Id: I8f14004a6b2e77366abad6e88786ea8941629020
1 file changed