Create an Expression without any operator

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## New Feature
Create an Expression without any operator

e.g.
```
class Expression : Value {
// ...other code
public Expression(@NonNull List array) : super(array) { }
// ...other code
```

## Why
In the iOS, we can create an Expression without any operator and there are iOS examples using that way.

```
// PointAnnotationClusteringExample.swift
// Create expression to get the total count of hydrants in a cluster
let sumExpression = Exp {
Exp(.sum) {
Exp(.accumulated)
Exp(.get) { "sum" }
}
1
}
```

However, we cannot do this right now for Android; hence there are examples of iOS cannot be ported to Android equivalently.

Start by tracing the Android Expression and Value APIs mentioned in the issue, then compare their construction with the PointAnnotationClusteringExample.swift example. Determine how an Expression with only a Value list should be represented and identify the relevant Android tests or examples to update. Done means the iOS usage can be ported equivalently to Android.

Written by the indexing model from the issue text.