Convert long_device_id field type from "bytes" to "string"

This CryptAuth backend CL converted the long_device_id field of the
ClientAppMetadata proto from type "bytes" to type "string":
https://critique.corp.google.com/#review/242222308.

Because Chrome OS has already been setting this field with a string,
this change should have no effect.

Note: This change is only necessary for v2 Enrollment; v1 code should
remain the same.

Bug: 899080
Change-Id: Icf4bd1c93b0c6aaad4a5245de8c882baa7b31f7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559757Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649247}

chromeos/services/device_sync/proto/cryptauth_client_app_metadata.proto

@@ -27,8 +27,13 @@ message ClientAppMetadata {
   string instance_id_token = 3;
   // Checkin android id of the device.
   fixed64 android_device_id = 4;
-  // Not all devices have device identifiers that fit in 64 bits.
-  bytes long_device_id = 5;
+  // Chrome and iOS use a UUID in the form xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
+  // Where x is a lowercase hex digit.
+  // For iOS, this UUID is the IDFV.
+  // For backward compatibility with some Chrome long_device_ids, this can also
+  // be something else.
+  // For the same reason, this field is case sensitive, even with valid UUIDs.
+  string long_device_id = 5;
 
   // Subgrouping of device features field. These help in targeting specific
   // class of devices, for ex: Tablets vs phones etc.