Allow setting LoadErrorHandlingPolicy for DRM session requests

[stevemayhew]

Use case description

Many of the Widevine proxy implementations require more elaborate retry mechanisms then the simple non-random timed backoff provided by the DefaultLoadErrorHandlingPolicy (for example, 429 back-off). This simple hook allows for implementations of them.

Proposed solution

See pull request #1272

Alternatives considered

Copy the entire DefaultDrmSessionManagerProvider and make this change for ourselves (ugly)

[stevemayhew]

@icbaker not sure where you guys are going with the factories around the drm package, as most are marked as @UnstableApi, which I understand as adding to these API's is pretty un-restricted, just don't depend on them to remain in place or produce a stable player.

We are using the DefaultDrmSessionManagerProvider as it allows for caching the DrmSession (critical for live playback with Widevine proxy)

[icbaker]

#1272 is merged, so closing this.