fix(select): in popup, hover and focus background colors are the same

[Splaktar]

PR Checklist

Please check that 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?

• In the md-select popup, hover and focus background colors are the same. This can cause confusion when navigating with both keyboard and mouse on desktop.

Issue Number:
Fixes #9851

What is the new behavior?

• use a similar approach to the one used by MDC. I.e. hover is 8% less opaque than focus.
  • the opacity values are slightly different because
    they use #000 and we use rgba(0, 0, 0, 0.87)

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

No tests as this is a CSS-only change.

[wagnermaciel]

lgtm