Reland "[Brightness] Fix small manual changes"

This reverts commit a2db90e3e3066a5e593d9a263f6e553e24ffaaf5.

Reason for revert: I reverted the original because the build failed after merging it. However, it now transpires that it had nothing to do with it. So, putting the original fix back in.

Original change's description:
> Revert "[Brightness] Fix small manual changes"
>
> This reverts commit cc8270a89cf8b42a6c48e38394d80779b7959362.
>
> Reason for revert: This may have caused problems with other integration tests. Investigating.
>
> Original change's description:
> > [Brightness] Fix small manual changes
> >
> > Small, manual changes that were smaller than the step size
> > for smooth changes were not performed.
> >
> > Test: New test added. Failed before fix.
> > Bug: b/178258598
> > Fixed: b/178258598
> > Change-Id: Ie0fc991ed5c15a009618e0c6660bf573e3879187
> > Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/474841
> > Commit-Queue: Kevin Twidle <[email protected]>
> > Reviewed-by: Lingxue Luo <[email protected]>
>
> [email protected],[email protected]
>
> Change-Id: I9efe3b7dc6b307bf083c8bc2c8b70ed6eb712c18
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: b/178258598
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/485749
> Reviewed-by: Kevin Twidle <[email protected]>
> Commit-Queue: Kevin Twidle <[email protected]>

Bug: b/178258598
Change-Id: I0758b59d0b828dd3d444a7e67d8c1d134a0eb9c7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/487786
Reviewed-by: Lingxue Luo <[email protected]>
Commit-Queue: Kevin Twidle <[email protected]>

• src/ui/bin/brightness_manager/src/control.rs[diff]