Refine media state machine methods.

* state( id ) becomes setState( id )
* get( id ) becomes state( id )
* state() stays the same
* previous() becomes lastState()

Props koopersmith
fixes #22652


git-svn-id: https://core.svn.wordpress.org/trunk@22952 1a063a9b-81f0-0310-95a4-ce76da25c4cd