Add gRPC internal headers module. am: 4a7a77cacb

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

Change-Id: Icdf941c3296b0c03c1f995849de5a089c607c9b2
Signed-off-by: Automerger Merge Worker <[email protected]>