adb: make `adb kill-server` wait for the server to die.

Make the host:kill service shutdown its socket on process exit, instead
of immediately. Also, unify the two 'kill-server' implementations and
hide _adb_connect.

Bug: http://b/37104408
Test: adb kill-server; adb start-server
Change-Id: I9475f5d084d5fb91d33e393f2fd4e34056613384