redfin: usb: add state of usb extcon to monitorffs

Monitorffs needs to check the state of usb extcon to pull up
USB gadget. Therefore, this patch add state of usb extcon and
send this information to monitorffs.

Bug: 173086594
Test: verified pass
Merged-In: I29df29efcf55118241f0ca7072b034c1fb5a37f2
Change-Id: I2c19f769991b014dfa7051e114abfecc40b90a6e
diff --git a/usb/UsbGadget.h b/usb/UsbGadget.h
index 93c7d66..32b173f 100644
--- a/usb/UsbGadget.h
+++ b/usb/UsbGadget.h
@@ -66,7 +66,8 @@
 using ::std::string;
 
 constexpr char kGadgetName[] = "a600000.dwc3";
-static MonitorFfs monitorFfs(kGadgetName);
+constexpr char kExtconUsbState[] = "/sys/class/extcon/extcon4/cable.0/state";
+static MonitorFfs monitorFfs(kGadgetName, kExtconUsbState);
 
 struct UsbGadget : public IUsbGadget {
     UsbGadget();