API cleanup: add NonNull annotation to Rollback API

Annotate the return values of RollbackInfo, PackageRollbackInfo and
RollbackManager API methods as NonNull.

Fixes: 126863286
Fixes: 126698364
Fixes: 126699071
Fixes: 126700362
Fixes: 126701437
Test: build
Change-Id: Id10a9edbf21a239f310ba1ee25ec96cf665c3d3e