Fix cancellation of MediaDataLoader jobs

We should only look at the key, otherwise we don't successfuly detect
duplicates. We also have apps bursting updates within a few ms as they
update metadata and we shouldn't spend battery loading artwork in that
case.

Bug: 358350077
Flag: com.android.systemui.media_load_metadata_via_media_data_loader
Test: atest MediaDataLoaderTest
Change-Id: Ie52fa043db48c58b8a9c23b45ed33ed5b4ab61ad