Makes files usage a bit more screenreader friendly

Adds a way to override the announcement on the ProgressBar component.
Also makes it so that the files usage percentage is reported with no more
than 2 decimal places

fixes CNVS-16863

Test Plan:
	- Enable New Files
	- Using a screenreader navigate to the Files Usage bar
	- Usage should be read with no more than 2 decimal places
	- NOTE: Please also check other uses of progress bars for potential regression.

Change-Id: If58f49545a76476ae68b3a4105b2a6cb6908111f
Reviewed-on: https://gerrit.instructure.com/44388
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>