Implement manifest location fallback logic in DashMediaSource.

This CL updates `DashMediaSource` to track multiple manifest locations and perform location fallback when errors are encountered during manifest loads, leveraging the `LoadErrorHandlingPolicy`.

* Remove `manifestUri` field and consolidate tracking under `manifestLocations` as the sole source of truth.
* Introduce `selectManifestLocation()` helper to dynamically resolve active, non-excluded locations.
* Update `onManifestLoadCompleted()` to check request relevance and refresh candidate locations from newly loaded manifest.
* Update `onManifestLoadError()` to interact with `LoadErrorHandlingPolicy`, exclude failing `serviceLocation` upon location fallback selection, and immediately start loading the next candidate.

PiperOrigin-RevId: 939961284