adb: wait for devices to come up instead of sleeping for 3s.

Replace a hard-coded 3 second sleep with logic to wait until we've
scanned USB devices once and they've all come online.

Before:
    adb shell true  0.00s user 0.00s system 0% cpu 3.047 total

After:
    adb shell true  0.00s user 0.00s system 9% cpu 0.041 total

Bug: http://b/37869663
Test: `time adb shell true` after adb kill-server
Change-Id: I251d42afb885908ed9d03167287594ea16650d3f
diff --git a/adb/transport.h b/adb/transport.h
index 8c15d66..e129355 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -198,6 +198,10 @@
 void kick_transport(atransport* t);
 void update_transports(void);
 
+// Iterates across all of the current and pending transports.
+// Stops iteration and returns false if fn returns false, otherwise returns true.
+bool iterate_transports(std::function<bool(const atransport*)> fn);
+
 void init_transport_registration(void);
 void init_mdns_transport_discovery(void);
 std::string list_transports(bool long_listing);