Get rid of using `MerkleStateRoot` features on initialization in `platform-core` code

[imalygin]

Currently there are three classes left in platform-core production code that are still using MerkleStateRoot class:

1. PlatformStateFacade. It casts State object to MerkleStateRoot to bypass normal access mechanism (via ReadableStates) because it's not initialized at the moment of invocation. Solution: we need to change the initialization so that we didn't have to do this direct access to Merkle tree node by index
2. SignedStateFileReader. registerServiceState uses MerkleStateRoot#putServiceStateIfAbsent.
   There are two possible soltions:

• putServiceStateIfAbsent can be elevated to State interface. This will require moving StateMetadata
• registerServiceState can have a callback parameters passed by the hedera-app code

3. SignedStateFileWriter casts state to initialize time field:

        if (state instanceof MerkleStateRoot merkleStateRoot) {
            merkleStateRoot.setTime(platformContext.getTime());
        }

It can be fixed by setting the time where it's more appropriate.