Allow users to set overrides for MediaElement instances by always passing `_wpmejsSettings` instead of just `_wpmejsSettings.pluginPath`.

Props Otto42 for the initial patch.
Fixes #25243.


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git-svn-id: http://core.svn.wordpress.org/trunk@28198 1a063a9b-81f0-0310-95a4-ce76da25c4cd