Replace explicit use of VectorMTJ by alias ActsTrk::TrackStateBackend

Tomasz Bold requested to merge tbold/athena:mtj-typedef-cont-1 into master

This is followup of !63356 (merged). It replaces use of explicit type from ACTS by an alias defined in Athena ActsTrk::TrackStateBackend.

After that one we should be only one step away from trying Athena backend just by recompiling the Reconstruction/Acts (+ modifications to how the MTJ gets constructed).

FI @pagessin @cvarni @ncalace @adye @mwolter (and please tag further if needed)

BTW. I think a similar action is needed for Acts::VectorTrackContainer

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