Follow-up of aosp/3044634 for root id

With aosp/3044634 root id is brought back to native but
was still missing in the response, adding it.

This doesn't need to be flagged as the root id passed in
the callback of SetBrowsedPlayerResponse before the
refactor was already "".

Bug: 297843126
Flag: Exempt trivial
Test: atest BluetoothInstrumentationTests
Change-Id: I5efbe012038a75d119fcd30977aa72d3550588f8