fix(list): case where user is unable to interact w/ secondary actions

[Splaktar]

PR Checklist

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• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

If the secondary actions' template was provided by a directive, then the actions would be behind the primary button. This meant that tooltips and clicks were not activated.

Issue Number:
Fixes #9676

What is the new behavior?

Secondary action tooltips and clicks are activated when the template is provided by a directive.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

#9676 proposed adding a z-index to the md-secondary-container, but the container did not have a position. By the spec, z-index should only be on "positioned" elements. Adding positioning resolved the issue w/o needing to define a z-index.