[BUGFIX] Re-read extListArray in em at runtime

The introduction package loads multiple extensions in one go through the
em API enableExtension(). The refreshGlobalExtList() method should reset
the extListArray to new values written to LocalConfiguration to
recognize if many extensions are loaded in one request.

Change-Id: Ieb4481e3d04a4407ba31f3c64f71c5fb5b03e344
Resolves: #39237
Related: #39112
Releases: 6.0
Reviewed-on: http://review.typo3.org/13038
Reviewed-by: Wouter Wolters
Reviewed-by: Georg Ringer
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel