[stts] Add representative target id to device info specifics

The representative target id (RTID) is required to send notifications
to specific devices through Chime. This CL provides the RTID to the
device info specifics so that push notifications can be sent to a
user's synced device.

This will be submitted before its downstream compliment:
crrev.com/i/7454856

Bug: 343495515
Low-Coverage-Reason: TESTS_IN_SEPARATE_CL
Change-Id: Ibc05ab926a6f98ca1689097dcaa380e0e21e609a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5677790
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Daniel White <danieltwhite@google.com>
Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Cr-Commit-Position: refs/heads/main@{#1327730}