Don't reference RollbackManagerService directly (2/n)

This is an effort to modularize RollbackManager.

As we move RollbackManagerService to apex/, SystemServer can't reference
RollbackManagerService directly. Let's use the class name to start the
service.

(Cherry-picked from 41a5d353)

Bug: 150347230
Test: m
Merged-In: Ied68ad031f84ea4a416ad779e9ddf4cc912bd7b9
Change-Id: Ied68ad031f84ea4a416ad779e9ddf4cc912bd7b9