[SPARK-3443][MLLIB] update default values of tree:

Adjust the default values of decision tree, based on the memory requirement discussed in https://github.com/apache/spark/pull/2125 :

1. maxMemoryInMB: 128 -> 256
2. maxBins: 100 -> 32
3. maxDepth: 4 -> 5 (in some example code)

@jkbradley