Fix several bugs (duplicate load media, media listeners bug, etc)

Hi. This pull request has several arrangements.
It has been tested with Chrome sender apps from Google as "hello-video" and "cast-videos".
In "chrome.cast.js" line 531 caused to be invoked twice the load of video.
Media update listeners were not recovering the isAlive parameter.
Have adapted the classes methods to fit the parameters expected.
In "ChromecastSession.java", MediaChannelResult callback is not where it should invoke "onMediaLoaded" listener but in the onSuccess of your getStatus. Now "mediaLoaded" has an active session.

Sorry my bad english, hope I explained clearly.

Thanks for your great plugin.