versioner: Add 28 to version list

This commit adds 28 to version list so that we can pass
slow_preprocessor_idempotence.  If 28 is not in the version list,
versioner will add another `#ifdef` guard and fail the test.

Bug: 113263746
Test: ./bionic/tools/versioner/run_tests.py
Change-Id: I3f21fc71ffd9e55326c4587bbc5e689eb72596b6