Unify the flow of enabling rollbacks (2/n)

* Remove the workaround in makeVerificationParamsLocked() since now
  we have only once place to enable rollbacks.
* Listen to the broadcast to enable rollbacks for both staged and
  non-staged sessions.
* notifyStagedSession() becomes a getter.
* SessionCallback cares about parent sessions only which tell if a
  session is installed successfully or not.

Bug: 183466803
Test: atest RollbackTest CtsRollbackManagerTestCases
Test: atest MultiUserRollbackTest CtsRollbackManagerHostTestCases StagedRollbackTest

Change-Id: I2a6b58f124b70fac731c68ad2b91403586c26f19