fix(fab-speed-dial): opens when trigger button is disabled

[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?

• md-fab-speed-dial still triggers action when button inside of md-fab-trigger is disabled
• md-fab buttons have box-shadow when disabled

Fixes #9467

What is the new behavior?

• when the trigger is clicked, verify that the trigger button isn't disabled
  before toggling the open state
• style the disabled speed dial trigger fab button using the current theme
  to support both light and dark themes
• fix issue where the disabled background (mostly transparent)
  allowed opaque md-fab-action-items to fade through
• disabled fab buttons now have no box-shadow which is consistent
  with Angular Material
• fix basic usage demo to not have a black hover/focus background
  as that doesn't work at all on dark theme and was confusing in light theme
• add test for when the click occurs on the md-icon instead of the md-button
• remove duplicate $button-fab-width variable

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Before

After

Dark theme

[mmalerba]

LGTM