Use ActivitySerializationContext when describing a Standalone Activit… #5460

Job	Run time
check-protos	8m 24s
cloud-test	2m 2s
test-latest-deps	10m 51s
features-tests / test	9m 0s
build-lint-test (3.14, ubuntu-arm)	12m 39s
build-lint-test (3.14, windows-latest)	21m 43s
build-lint-test (3.14, ubuntu-latest)	21m 31s
build-lint-test (3.14, macos-arm)	11m 32s
build-lint-test (3.10, macos-arm)	10m 11s
build-lint-test (3.10, ubuntu-latest)	15m 42s
build-lint-test (3.10, windows-latest)	19m 3s
build-lint-test (3.10, ubuntu-arm)	12m 16s
	2h 34m 54s

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