libsnapshot: Add prolog to RemoveAllUpdateStates.

Add an optional prolog arg (function<bool()>) that is invoked
before snapshots are deleted and update state set to none.

This allows update_engine to delete markers before deleting snapshots
to avoid depending on the erroneous markers. Otherwise, if update_engine
delete markers after libsnapshot deletes update states, the device could
technically get into a state where update_engine thinks the update has
been applied, but snapshots are gone.

Bug: 147696014
Test: libsnapshot_test

Change-Id: I71bfc04a81ea4f94b3072558be50d2f80565113e
Merged-In: I71bfc04a81ea4f94b3072558be50d2f80565113e