Pin third_party/upb to C17. am: 8e1018b031 am: 512eb73998

Original change: https://android-review.googlesource.com/c/platform/external/grpc-grpc/+/3320794

Change-Id: Iff6d1d3b8c95da60c5884996ee03f9c39aa355a9
Signed-off-by: Automerger Merge Worker <[email protected]>