DO NOT MERGE: Prefill the cache before trying to instantiate the media extractor.

The latter is an operation that otherwise could block on the datasource
for a significant amount of time. During that time we'd be unable to
abort the preparation phase without this prefill.

Change-Id: I3bc889b264f599bfd5c2bbdf48b88ccb55d86172
related-to-bug: 3362836