Revert "Revert "Change timestamp to -stubs.srcjar""

This reverts commit 9ff085f9.

Reason for revert:
Change timestamp to -stubs.srcjar 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: I40ba48fa40b08b1a20b718fbcd5c260450397fc7