adb: retry connecting disconnected emulators instead of always looping.

Previously we loop through local ports every second, this patch improves
the strategy by retrying only just disconnected emulators.

Bug: 26468076
Bug: 19974213
Bug: 22920867

Change-Id: I43ccb746922d104202b0f81a3d163d850bbc890e
diff --git a/adb/adb.h b/adb/adb.h
index cb38e61..9227eb1 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -188,7 +188,7 @@
 
 
 void local_init(int port);
-void local_connect(int port);
+bool local_connect(int port);
 int  local_connect_arbitrary_ports(int console_port, int adb_port, std::string* error);
 
 // USB host/client interface.