Use ActivitySerializationContext when describing a Standalone Activity #5458

Job	Run time
test-latest-deps	10m 57s
build-lint-test (3.10, ubuntu-latest)	14m 29s
cloud-test	2m 9s
check-protos	8m 55s
build-lint-test (3.10, macos-arm)	9m 50s
build-lint-test (3.14, ubuntu-arm)	11m 52s
features-tests / test	8m 32s
build-lint-test (3.14, ubuntu-latest)	18m 59s
build-lint-test (3.10, ubuntu-arm)	14m 47s
build-lint-test (3.10, windows-latest)	21m 21s
build-lint-test (3.14, macos-arm)	7m 12s
build-lint-test (3.14, windows-latest)	21m 14s
	2h 30m 17s

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