Make drop consistent with paste for multiple text items in a ClipData

A ClipData can contain multiple items, for example:
```
ClipData clip = ClipData.newPlainText("label", "ONE");
clip.addItem(new ClipData.Item("TWO"));
clip.addItem(new ClipData.Item("THREE"));
```

When a clip like this is pasted, the TextView impl inserts a newline
between each item. When the same clip is inserted via drag-and-drop,
no newlines are inserted.

This change makes the behavior of drop and paste consistent by updating
drop to match the paste behavior (since pasting has been around longer
and is more common).

Bug: 187428338
Test: atest CtsWidgetTestCases:TextViewReceiveContentTest
Change-Id: I08f30e619b858a4586ab9b4ed287be28b218231e