bramble: 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: I2996e4e9659af894931e0e4300d32143b6594667
Change-Id: I56d55674d31dfb4984600a89ae53424f013f184e
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 {