[lowpan] meshcop `rv` field workaround

We are acquiring meshcop txt entries from OpenThread, it is expected to
have a `rv` field but it's not presented in the current OpenThread
commit used by F28.

Update border agent logic to add the `rv` field if OpenThread
doesn't expose it.

Bug: 443908945
Change-Id: I5f1f668e027a4e2ff27a70ccb6bc3b0c05af16c7
1 file changed