adb: switch reconnect_queue to std::set.

We can't remove from the middle of a priority_queue, which a followup
commit wants to do, so switch to std::set, with the side benefit of
making operator< point the right direction.

Test: mma
Test: ./test_adb.py
Change-Id: I784c1dcc91f0a9cf760e9fa1710202e37e85432b