libsnapshot: Re-expose InitiateMerge and ProcessUpdateState

These functions are used directly in update_engine now that
it is responsible for initiating the merge. update_engine has
its own message loop, so WaitForMerge / InitiateMergeAndWait
becomes useless and are deprecated.

Bug: 147696014
Test: compiles

Change-Id: Id8cb14f5f18b4893e2fc3d0d2b374a867a220e6f

fs_mgr/libsnapshot/include/libsnapshot/snapshot.h

@@ -140,7 +140,6 @@ class SnapshotManager final {
     // Before calling this function, all snapshots must be mapped.
     bool FinishedSnapshotWrites();
 
-  private:
     // Initiate a merge on all snapshot devices. This should only be used after an
     // update has been marked successful after booting.
     bool InitiateMerge();
@@ -172,7 +171,6 @@ class SnapshotManager final {
     UpdateState ProcessUpdateState(const std::function<void()>& callback = {},
                                    const std::function<bool()>& before_cancel = {});
 
-  public:
     // Initiate the merge if necessary, then wait for the merge to finish.
     // See InitiateMerge() and ProcessUpdateState() for details.
     // Returns: