Improve NO_PLAYABLE_REPRESENTATION behavior for the v4

[peaBerberian]

In the v3.x.x, the impossibility to play any Representation from a chosen track due to non-usable keys, lead to an error with the NO_PLAYABLE_REPRESENTATION error code .

This seemed sensible at the time, but we're now encountering use cases where it would be more sensible to change the current track instead, to one that perhaps has decipherable Representation(s).

The main example would be to have a separate video track linked to to another dynamic range (e.g. an HDR and a SDR track) each with different security policies (tracks with higher dynamic range would have more drastic security policies for example). Here, I would guess that an application would prefer that by default we switch to the SDR video track if no
Representation in the HDR one is decipherable, instead of just stopping playback with a
NO_PLAYABLE_REPRESENTATION error.

This is sadly not something we can do easilty in a v3.x.x because it would imply breaking changes (such as switching to the RELOADING state during the track change without a supplementary API), but that's definitely something we would want to implement in the v4.

Because an application might still want to be notified or even stop playback by itself when the chosen track has no playable Representation, I added the no-playable-Representation reason to the trackUpdate event, which indicates that the current track for any Period of the current content was updated due to this situation.

The NO_PLAYABLE_REPRESENTATION error is still thrown, only now it is when no Representation of all tracks for the given type are decipherable.