Expose license expiration times

[joeyparrish]

Shaka Player should expose license expiration times, both for playing content and for content stored offline.

For playing content, we already have this information in DrmEngine. It should be exposed through a method on Player. It might also be useful to fire an event from Player when the expiration time changes.

For offline content, we could get the expiration time by loading the stored sessions. Ideally, we would store this when the content is initially stored, rather than loading all stored sessions when the application asks for a content list.

For offline, we should start with a change to store the expiration time when we store the content. As persistent licenses are currently only supported by Chrome on ChromeOS, it is unlikely that there is a strong need to update existing stored content with expiration times. If the community requests an update path, we can add one later.

[joeyparrish]

Adding this optimistically to the v2.1.0 milestone. If we finish HLS before we finish this, we'll bump this feature to the next release.