Make the InkResponse's focus highlight honor the radius parameter

[darrenaustin]

Description

Currently, the highlight used on an InkResponse when it is focused is a circle with a hard coded radius of 35 (Material.defaultSplashRadius). While a good default, it may not be appropriate for all use cases. This PR has the response use the radius parameter for this if it is given. If not it reverts to Material.defaultSplashRadius as normal.

Tests

I added a test 'ink response uses radius for focus highlight' to ink_well_test.dart.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them?

• No, no existing tests failed, so this is not a breaking change.

[HansMuller]

LGTM

[gspencergoog]