message: buffer impl: add cast for android compilation
  Commit Message: add cast for android compilation
  Risk Level: low using the expected type for the constructor as the static_cast type.
  Testing: local build of envoy mobile for android. CI
diff: @@ -40,7 +40,7 @@ public:
  // SliceData
  absl::Span<uint8_t> getMutableData() override {
  RELEASE_ASSERT(isMutable(), "Not allowed to call getMutableData if slice is immutable");
  - return {base_ + data_, reservable_ - data_};
  + return {base_ + data_, static_cast<absl::Span<uint8_t>::size_type>(reservable_ - data_)};
  }
  /**
  