[SPARK-18643][SPARKR] SparkR hangs at session start when installed as a package without Spark

## What changes were proposed in this pull request?

If SparkR is running as a package and it has previously downloaded Spark Jar it should be able to run as before without having to set SPARK_HOME. Basically with this bug the auto install Spark will only work in the first session.

This seems to be a regression on the earlier behavior.

Fix is to always try to install or check for the cached Spark if running in an interactive session.
As discussed before, we should probably only install Spark iff running in an interactive session (R shell, RStudio etc)

## How was this patch tested?

Manually

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16077 from felixcheung/rsessioninteractive.