Make message format change backwards compatible.

Test: create a reference dump for libc++ with the old format, copy it to
prebuilts/abi-dumps/vndk, run mm -j64 to invoke header-abi-diff,no protobuf
TextFormat errors reported.

Bug: 62463617

Change-Id: I1d9f952a77a3d5f5276773078876dde17d360ecc
5 files changed