fix(material/progress-bar): fix screenreader not reading aria-label

[annieyw]

Fixes issue with NVDA/FIreFox not reading out aria-label (when set)
Fixes #15017

Note that this does not fix the issue for JAWS/FireFox (FreedomScientific/standards-support#354)

[annieyw]

I tested this in the dev app and the value updates fine but for some reason the unit test is failing, any ideas?

[jelbourn]

Commenting here for posterity: Annie and I chatted about this change and we're going to look into changing to a solution that will let people keep using the actual ARIA attributes rather than introducing new properties (which we can then apply to other components)

[annieyw]

Update:
Instead of adding an accessible label we have decided to simply set tabindex to -1 for aria-label to work with most screen reader/ browser combos. (Note that there is a known issue with JAWS/FireFox.)

[annieyw]

added comments to explain change and note about FireFox/JAWS issue

[jelbourn]

LGTM

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