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.cpp b/adb/transport.cpp
index cc8c162..20de642 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -400,8 +400,27 @@
     return &tracker->socket;
 }
 
+// Check if all of the USB transports are connected.
+bool iterate_transports(std::function<bool(const atransport*)> fn) {
+    std::lock_guard<std::mutex> lock(transport_lock);
+    for (const auto& t : transport_list) {
+        if (!fn(t)) {
+            return false;
+        }
+    }
+    for (const auto& t : pending_list) {
+        if (!fn(t)) {
+            return false;
+        }
+    }
+    return true;
+}
+
 // Call this function each time the transport list has changed.
 void update_transports() {
+    update_transport_status();
+
+    // Notify `adb track-devices` clients.
     std::string transports = list_transports(false);
 
     device_tracker* tracker = device_tracker_list;