Add gRPC internal headers module. am: 4a7a77cacb

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/grpc-grpc/+/22982889

Change-Id: I9b60abfd986727f8281058a04ecb7203a2c1bac6
Signed-off-by: Automerger Merge Worker <[email protected]>