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
diff --git a/adb/adb_client.h b/adb/adb_client.h
index d07c1e9..fabec00 100644
--- a/adb/adb_client.h
+++ b/adb/adb_client.h
@@ -26,7 +26,9 @@
 // Connect to adb, connect to the named service, and return a valid fd for
 // interacting with that service upon success or a negative number on failure.
 int adb_connect(const std::string& service, std::string* _Nonnull error);
-int _adb_connect(const std::string& service, std::string* _Nonnull error);
+
+// Kill the currently running adb server, if it exists.
+bool adb_kill_server();
 
 // Connect to adb, connect to the named service, returns true if the connection
 // succeeded AND the service returned OKAY. Outputs any returned error otherwise.