Support rollback for rebootless apex (2/n)

The code is already there. We just need to remove
the restriction to enable rollback support.

* Note only the RETAIN policy is supported for rebootless APEX.
* Note this CL breaks CtsInstallHostTestCases. We will fix it
  in the next CL.

Bug: 195517333
Test: atest StagedRollbackTest

Change-Id: Ib218186fe1448f1f316d2c3b895636efc657ee0c