b/12068020 add nonce to InputDeviceDescriptor. Do not merge

This is a cherry-pick of https://googleplex-android-review.git.corp.google.com/#/c/401795/

Adds a nonce field to allow generation of unique descriptors for
input devices.

Change-Id: Ie82bfb728bea95792b3b138dd3974f8b0aff63e0

include/input/InputDevice.h

@@ -46,6 +46,11 @@ struct InputDeviceIdentifier {
     // Ideally, the way this value is computed should not change between Android releases
     // because that would invalidate persistent settings that rely on it.
     String8 descriptor;
+
+    // A value added to uniquely identify a device in the absence of a unique id. This
+    // is intended to be a minimum way to distinguish from other active devices and may
+    // reuse values that are not associated with an input anymore.
+    uint16_t nonce;
 };
 
 /*