fix(select): height changes when disabled

[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?

• md-select's height changes when disabled

Issue Number:
Fixes #11812

What is the new behavior?

• md-select's height no longer changes when disabled
• use the same approach for md-select's disabled border background-position as md-input
• md-select inside of an md-input-container uses more md-input SCSS variables
  • this allows it to better match the layout and dimensions of other md-input-container elements
• add new $select-border-width-default SCSS variable

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This change should not break any apps, but it may allow them to remove some CSS hacks to fix the previous issue with the selects loosing 1px of height when disabled. If there are screenshot tests involved, they may fail for disabled inputs due to the 1px change in position, but this new position should be the correct position and matches the non-disabled position.

We'll try this out in an RC and determine if this will be problematic for production apps or not. If it turns out to be, we'll revert this and push it to 1.2.0.

[mmalerba]

LGTM