[QuickStart]: Send sessionId as string in message payload

Source phone expects a 64-bit ID, but this isn't supported by JSON.
Alternatively, send the 64-bit ID as a string so that the phone can
parse it. This is how the current phone-to-phone flow works.

Test: updated existing unittests, manually tested with atlas and pixel8
Change-Id: I4a59ec6c651e420dfbce62955795ba37d932a9b5
Bug: b/307580682
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5411374


Reviewed-by: Michael Hansen <hansenmichael@google.com>
Commit-Queue: Brittany Hartmire <bhartmire@google.com>
Cr-Commit-Position: refs/heads/main@{#1281295}