[tools] Add column header for emulators information

[Anas35]

Add column information as table header for flutter emulators command.

Before:

2 available emulators:

Pixel_3_API_30   • Pixel 3 API 30   • Google • android
Resizable_API_33 • Resizable API 33 • Google • android

To run an emulator, run 'flutter emulators --launch <emulator id>'.
To create a new emulator, run 'flutter emulators --create [--name xyz]'.

You can find more information on managing emulators at the links below:
  https://developer.android.com/studio/run/managing-avds
  https://developer.android.com/studio/command-line/avdmanager

After:

2 available emulators:

Id               • Name             • Manufacturer • Platform

Pixel_3_API_30   • Pixel 3 API 30   • Google       • android
Resizable_API_33 • Resizable API 33 • Google       • android

To run an emulator, run 'flutter emulators --launch <emulator id>'.
To create a new emulator, run 'flutter emulators --create [--name xyz]'.

You can find more information on managing emulators at the links below:
  https://developer.android.com/studio/run/managing-avds
  https://developer.android.com/studio/command-line/avdmanager

fixes #140656

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, including Features we expect every widget to implement.
• 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 this PR is test-exempt.
• All existing and new tests are passing.

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

[christopherfujino]

LGTM

[eliasyishak]

nice, lgtm