fix(fab-speed-dial): action items are in tab order when closed

[Splaktar]

PR Checklist

Please check your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

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?

• action item buttons in a closed md-fab-speed-dial are part of the tab order
• this can also cause tooltips to appear (if the buttons have tooltips) that aren't connected to anything

Fixes #10101

What is the new behavior?

• apply tabindex="-1" to all md-buttons in the md-fab-actions

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[mmalerba]

LGTM