Generate prefix for DecimalType, fix the random generator of decimal

cc JoshRosen

Author: Davies Liu <davies@databricks.com>

Closes #7857 from davies/sort_decimal and squashes the following commits:

2433959 [Davies Liu] Merge branch 'master' of github.com:apache/spark into sort_decimal
de24253 [Davies Liu] fix style
0a54c1a [Davies Liu] sort decimal