Analytics onLoad events for HttpMediaDrmCallback requests

Use case description

An often significant delay for first frame with channel tunes occurs because of delays from the DRM License Server key or provisioning requests, it would be nice to have analytics events for these.

Proposed solution

The standard MediaSourceEventListener report details for DATA_TYPE_DRM for fetches from EXT-X-KEY with an actual key URL, However the same level of details is missing from the DrmSessionEventListener events.

We could:

1. Add load statistics to the existing drmKeysLoaded event
2. Add two new events (drmKeyRequestStarted, drmKeyRequestCompleted)

Alternatives considered

Adding logging to the HttpMediaDrmCallback, this is simple but not as useful as a hook that allowed using AnalyticsListener to post the data to operational logging.