Clear flags when a rollback is committed.

This adds a call into RescueParty to clear flags relating to all
packages included in a rollback. This is done before the commit occurs
to ensure that the flags are cleared before the rolled-back version of
the package starts up.

Bug: 173120414
Test: atest RollbackManagerTest
Change-Id: I155cba0afc8a77517baa331b801c14ed11c50693