Ensures that hit testing only returns focusable nodes.

[abrush21]

This PR fixes an issue with the hit test algorithm whereby it would prematurely stop searching for nodes that enclose the hit point. If a child that encloses the touch point is not focusable, the algorithm should move on to the next child.

fxb/68331

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.
• The reviewer has submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[dnfield]

LGTM

[dnfield]

See also:

b/162065967
cl/353656895
b/177271344

[chinmaygarde]

@dnfield Can we land this?

[dnfield]

Yes - added the label