[SPARK-12148][SPARKR] SparkR: rename DataFrame to SparkDataFrame

## What changes were proposed in this pull request?

Changed class name defined in R from "DataFrame" to "SparkDataFrame". A popular package, S4Vector already defines "DataFrame" - this change is to avoid conflict.

Aside from class name and API/roxygen2 references, SparkR APIs like `createDataFrame`, `as.DataFrame` are not changed (S4Vector does not define a "as.DataFrame").

Since in R, one would rarely reference type/class, this change should have minimal/almost-no impact to a SparkR user in terms of back compat.

## How was this patch tested?

SparkR tests, manually loading S4Vector then SparkR package

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #12621 from felixcheung/rdataframe.