RollbackManager: Support userdata snapshot / restore.

The only significant missing piece of functionality is the
handling of credential encrypted data in the case where one
or more users haven't unlocked their device. This will be handled
in a follow-up change.

Bug: 112431924
Test: atest RollbackTest
Change-Id: I4582018337ce0344379f6f73d6fd6c9ce31c58a0