Add dumpsys for RollbackManager

Add a dump method to RollbackServiceManagerImpl that dumps the rollback
info of all rollbacks. The info includes:
        * Rollback Id
        * Rollback State
        * Staged session Id (if any)
        * Is rollback staged
        * Packages
        * Cause Packages (if any)
        * Committed session Id (if committed)
Example of format:
  868886202:
    -state: committed
    -timestamp: 2019-03-11T15:53:30.101Z
    -stagedSessionId: 1452111490
    -packages:
      com.android.tests.rollback.testapex 2 -> 1
    -causePackages:
    -committedSessionId: 1263657628

Fixes: 128404022
Test: adb shell dumpsys rollback
Change-Id: I624bfe465731d8e29fe65cd9f2587e59e348997a