libsnapshot: RemoveUpdateState on rollback.

If rollback is detected in ProcessUpdateState, call
RemoveUpdateState and return UpdateState::Cancelled. Now that
update_engine is reponsible for initiating the merge, it can react to
this state and clean up markers appropriately.

Test: libsnapshot_test
Test: apply OTA, manually rollback (by setting the active slot), then
  inspect /metadata/ota as well as /data/misc/update_engine/prefs.

Bug: 147696014

Change-Id: Ibfee11fb50e4f4fb7c6cf02b4921b35e77b8f5a5
Merged-In: Ibfee11fb50e4f4fb7c6cf02b4921b35e77b8f5a5