make ROOTTREE default storage for metadata instead of ROOTTREEINDEX
Follow-up from !67839 (merged), in particular !67839 (comment 7448149).
Bonus: fix a bug in xAOD::Utils::getFirstBranchMatch
(909bbd78, e34b7405), see: !67972 (comment 7449441)
Edited by Maciej Pawel Szymanski