[angular] fixes for custom ids

[mshima]

trackById and getSelected functions used to assume that id always exists.
Refactor to trackEntityByField format and getSelectedEntity.

Related to #7112.

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[kaidohallik]

LGTM.

The only concern is that as now more than 1 track / getSelected is generated and those are uncovered by unit tests then test coverage decreases.
So if possible then it would be good to add unit tests for track / getSelected.

[mshima]

@kaidohallik done, I will merge so I can rebase the other PR.
Don't hesitate to ask for changes after merged.