Revert "Use VERSION_CODES.S instead of VERSION_CODES.R + 1"

This reverts commit 7f13ec41.

Reason for revert:
1. The BUILD_VERSIONS.S is 1000, and thus it will prevent
   the code from requiring new algorithms on devices whose
   first sdk is 31 (e.g. cuttlefish), though these devices
   should be treated as first launched with SDK S.
2. It will break #testValidationForAlgosAddedInS, because
   the test code is using BUILD_VERSIONS.R to gate the test,
   which is inconsistent with the implementation.

Bug: 181887451
Test: atest IpSecAlgorithmTest
Change-Id: I5cd717c5ebd6086ae5cf9abf76311ae4fca0c6e9