Bump protobufs to 3.9.2

[rogerhu]

v3.9.0 has a known issue with < Android O versions. Wait to bump to 3.11.0 when it's ready.

This fixes warnings related to protocolbuffers/protobuf#3781

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.google.protobuf.UnsafeUtil (file:/mnt/nfs/shared-cache/bundletool-all-0.10.2.jar) to field java.nio.Buffer.address
WARNING: Please consider reporting this to the maintainers of com.google.protobuf.UnsafeUtil
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

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