Revert "Revert "Add offline/online-sdk-docs in Soong.""

This reverts commit 3c47f131.

Reason for revert:
Add offline/online-sdk-docs in Soong, attempt 2.

This is the second attempt to merge this CL topic.
The first attempt failed because <sdk.atree failed to find docs/*.txt files>.
This CL fixes this by <go/ag/4636078, go/ag/4636072>

Test: m -j out/target/common/docs/offline-sdk-timestamp
Bug: b/70351683
Change-Id: I338abfcfd3a15ce54436732f0d70c693a7e063c5