fix(material/slider): some screen readers announcing long decimal values - #20760

…ues (#20760)

It looks like some screen readers announce the value of a slider by calculating the
percentage themselves using the `aria-valuemin`, `aria-valuemax` and `aria-valuenow`.
The problem is that they don't round down the decimals so for a slider between 0 and 1
with a step of 0.1, they end up reading out values like 0.20000068. These changes work
around the issue by setting `aria-valuetext` to the same value that we shown in the thumb
which we truncate ourselves.

Fixes #20719.

(cherry picked from commit 19f5f5c)

…imal values (angular#20760)"

This reverts commit 19f5f5c.

Reverting this for two reasons:
* the truthiness check is a bit too broad, preventing someone
  from setting the empty string or zero
* I'd prefer to have the input named `valuetext` rather than `aria-valuetext`
  since it's potentially confusing without supporting the `attr.`
  (I know we do this elsewhere, but I'd like to avoid more use of
  this pattern)

…imal values (#20760)" (#20849)

This reverts commit 19f5f5c.

Reverting this for two reasons:
* the truthiness check is a bit too broad, preventing someone
  from setting the empty string or zero
* I'd prefer to have the input named `valuetext` rather than `aria-valuetext`
  since it's potentially confusing without supporting the `attr.`
  (I know we do this elsewhere, but I'd like to avoid more use of
  this pattern)

…imal values (#20760)" (#20849)

This reverts commit 19f5f5c.

Reverting this for two reasons:
* the truthiness check is a bit too broad, preventing someone
  from setting the empty string or zero
* I'd prefer to have the input named `valuetext` rather than `aria-valuetext`
  since it's potentially confusing without supporting the `attr.`
  (I know we do this elsewhere, but I'd like to avoid more use of
  this pattern)

(cherry picked from commit cc9758d)