Tweak "Added in API" for symbols not in a finalized dessert

Symbols can

  - exist in the Android SDK (@apiSince),
  - one or more extension SDKs (@sdkExtSince),
  - or any combination thereof (only @apiSince, @apiSince + @sdkExtSince, only @sdkExtSince)

Update doclava to procude the correct "Added in API" text for symbols
that only exist in extensions SDKs (only @sdkExtSince); the generated
text is

  Added in <extension> <version>

Bug: 259367545
Test: m docs # manually inspect out/target/common/docs/offline-sdk-docs-docs.zip
Change-Id: Ia7bdd5e5a6b9659ef546fbd82f3a254accc63aaa
1 file changed