Fix resource-loading code for TransitionSet

The inflation code in TransitionInflater was using the wrong
tag ("set") for TransitionSet. This fix corrects that problem
(changing it to "transitionSet") and documents the correct
tag in the TransitionSet javadocs.

Issue #11085279 Transitions: transition sets loaded from resources don't work

Change-Id: I8aaea9f31bbe368cffcca63d4eb6a5ec06c3ce7b