Make RollbackManager @SystemApi.

Require callers hold a new MANAGE_ROLLBACKS permission to interact with
the RollbackManager.

Bug: 112431924
Test: atest RollbackTest, with new test for permissions added,
      and with selinux in permissive mode.
Test: atest CtsPermission2TestCases:PermissionPolicyTest
Change-Id: I73f4f3457d85be580670cd69c89066d2cc348186