Support cast live seekable ranges and seek to the live position.

[dfoelster]

Currently the seekable range of live streams is not exposed in the CastPlayer.
The duration is always reported as C.TIME_UNSET.

This PR checks the media status of the active item for a LiveSeekableRange and exposes it over the CastTimeline.
Because the Timeline is only updated on status changed callbacks from the cast sdk (which are not send for livestream range changes) it would be outdated immediately.
To avoid this issue the CastTimeline stores it's creation timestamp and adjusts the reported window according to the elapsed time. This is similar to the internal behavior of the RemoteMediaClient.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[microkatz]

@dfoelster

Hi you'll need to sign the CLA agreement before we can review your pull request. Thank you!

[albinus-sc]

@microkatz @dfoelster Hey, this pull request seems like it might solve some of the issues we are facing regarding casting live content with a buffer. Any updates on this?

[microkatz]

@albinus-sc

Thank you for +1 the PR/feature. There are not any updates on this as @dfoelster has not signed the CLA agreement so we have not reviewed it. If you would like to take on the PR yourself to support adding the feature then you may! Feel free to make a new PR with the feature and some tests and we'd be happy to review! If there are specific bugs separate than this PR then feel free to file tickets for those as well.

[albinus-sc]

@microkatz I'm afraid I don't understand the changes well enough to create my own PR, let alone create tests for it.

I just wanted to let you know that this is a problem we're struggling with and if you at some point have the time to look at it yourself it would be very much appreciated!