Flattening server-side EditorRow API (#13334)

This commit just flattens the EditorRow class into Grid without changing
any behavior. isEditing() was changed to isEditorRowActive() since
isEditorRowEditing() didn't make any sense. Some javadocs have also been
slightly tweaked.

Change-Id: I7c266328ea93bde3e476bfb6e4d594469de26c76