Add counts to MusicMediaContainer

[seehamrun]

We were trying to figure out why no counts were written for MUSIC jobs even though the AppleMusicImporter sets the counts here:

data-transfer-project/extensions/data-transfer/portability-data-transfer-apple/src/main/java/org/datatransferproject/datatransfer/apple/music/AppleMusicImporter.java

Lines 91 to 97 in 822e0e7

	final Map<String, Integer> counts =
	new ImmutableMap.Builder<String, Integer>()
	.put(AppleMusicConstants.PLAYLISTS_COUNT_DATA_NAME, playlistsCount)
	.put(AppleMusicConstants.PLAYLIST_ITEMS_COUNT_DATA_NAME, playlistItemsCount)
	.build();
	return ImportResult.OK
	.copyWithCounts(counts);

And this should be preserved the same way photos counts are preserved

Upon investigating it looks like we completely disregard the counts returned from the importer in favor of the counts that are with the original exported data here:

data-transfer-project/portability-transfer/src/main/java/org/datatransferproject/transfer/CallableImporter.java

Line 80 in 822e0e7

result = result.copyWithCounts(data.getCounts());

Which works for Media jobs because the MediaContainerResource overrides the getCounts method here:

data-transfer-project/portability-types-common/src/main/java/org/datatransferproject/types/common/models/media/MediaContainerResource.java

Lines 112 to 118 in 822e0e7

	public Map<String, Integer> getCounts() {
	return new ImmutableMap.Builder<String, Integer>()
	.put(ALBUMS_COUNT_DATA_NAME, albums.size())
	.put(PHOTOS_COUNT_DATA_NAME, photos.size())
	.put(VIDEOS_COUNT_DATA_NAME, videos.size())
	.build();
	}

This PR adds a getCounts method to the MusicMediaContainer and updates the strings that are used in the AppleMusicImporter to be the central strings instead (this prevents a mismatch in the strings that are being used)

This PR just makes things operate as they are operating for photos/media, however the larger question is in the callable importer which counts we want to use (the ones from the result of the import or the ones from the data) and that is likely a larger discussion to be had.

This also means that Apple->YTM transfers now have count information as well since the GoogleMusicImporter does not return counts