Update nullability of extension versions in Rollback.

This updates nullability annotations in Rollback and RollbackStore as
suggested in comments on ag/11256434.

Bug: 152737927
Test: atest RollbackUnitTest
Test: atest RollbackTest
Test: atest RollbackStoreTest
Change-Id: Ib0973378fdd24ce275b0f3e1a687f9161d3c0e7d
3 files changed