fix(list): case where user is unable to interact w/ secondary actions (#11539)
fix(list): case where user is unable to interact w/ secondary actions (#11539)

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## PR Checklist
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- [x] The commit message follows [our guidelines](https://github.com/angular/material/blob/master/.github/CONTRIBUTING.md#-commit-message-format)
- [ ] 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?
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```
[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
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[ ] Infrastructure changes
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## 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.

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Issue Number: 
Fixes #9676

## What is the new behavior?
Secondary action tooltips and clicks were are activated when the template is provided by a directive.

## Does this PR introduce a breaking change?
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[ ] Yes
[x] No
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## 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`.
clshortfuse/material,marekmicek/material,angular/material,Splaktar/material,miguelcobain/material,miguelcobain/material,clshortfuse/material,clshortfuse/material,angular/material,oostmeijer/material,clshortfuse/material,marekmicek/material,marekmicek/material,oostmeijer/material,bluebirdtech/material,bluebirdtech/material,miguelcobain/material,bluebirdtech/material,angular/material,bluebirdtech/material,Splaktar/material,miguelcobain/material,angular/material,oostmeijer/material,Splaktar/material,marekmicek/material,Splaktar/material,oostmeijer/material