[SPARK-19066][SPARKR] SparkR LDA doesn't set optimizer correctly

## What changes were proposed in this pull request?

spark.lda passes the optimizer "em" or "online" as a string to the backend. However, LDAWrapper doesn't set optimizer based on the value from R. Therefore, for optimizer "em", the `isDistributed` field is FALSE, which should be TRUE based on scala code.

In addition, the `summary` method should bring back the results related to `DistributedLDAModel`.

## How was this patch tested?
Manual tests by comparing with scala example.
Modified the current unit test: fix the incorrect unit test and add necessary tests for `summary` method.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #16464 from wangmiao1981/new.