Follow-up to 9f2cd68

Use `com.google.common.base.Supplier`, for compatibility with clients
that are using the Android version of Guava. Fixes e.g.

```
Exception in thread "main" java.lang.IncompatibleClassChangeError: Class com.google.common.base.Suppliers$NonSerializableMemoizingSupplier does not implement the requested interface java.util.function.Supplier
```

PiperOrigin-RevId: 563446857

core/src/main/java/com/google/googlejavaformat/Doc.java

@@ -19,6 +19,7 @@
 import static java.lang.Math.max;
 
 import com.google.common.base.MoreObjects;
+import com.google.common.base.Supplier;
 import com.google.common.base.Suppliers;
 import com.google.common.collect.DiscreteDomain;
 import com.google.common.collect.Iterators;
@@ -27,7 +28,6 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
-import java.util.function.Supplier;
 
 /**
  * {@link com.google.googlejavaformat.java.JavaInputAstVisitor JavaInputAstVisitor} outputs a