[CLEANUP] Improve code style for FormEngine::getTSCpid()

The PHPDoc is fixed. The method returns an array and not an integer.

Additionally the class variable that is used in this method for
storing the cached values is added as a property.

Releases: 6.2
Resolves: #56658
Change-Id: I6010cf97781001cd760c891da3df3bfb7a605b73