Add RollbackManagerInternal (4/n)

This is an effort to modularize RollbackManager.

AIDL interfaces (IRollbackManager in this case) can’t be used across
module boundaries. We need to add a wrapper so system_server classes
can depend on this wrapper instead of IRollbackManager.

See https://docs.google.com/document/d/1d60nWwTcwrLps8dWDkxtkcUxSNt6q1x9rgAKwG-xO9E/view#heading=h.vqwoptkbpscx
for more details.

Bug: 150347230
Test: atest StagedRollbackTest

Change-Id: Ie34489aa3276f75d192ac4a08112012ef7f94509