[Deep link] Use serviceConnection.rootPackageDirectoryForMainIsolate to get the project path

[hannah-hyj]

If the devtools is connected to a running flutter app, it should auto-load the project and deep links instead of let user manually input the project path.

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 the Flutter Style Guide recently, and have followed its advice.
• [ ] I signed the CLA.
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or there is a reason for not adding tests.

If you need help, consider asking for help on Discord.