Ok I found it. I will leave the anwser here in case it maybe helpful for anyone.
Actually genhtml is a perl script. If you are have git bash installed, you should have perl already. Try where perl on the cmd and it will show you the path.
For me it was at C:\Program Files\Git\usr\bin\perl.exe
Now create a file called genhtml.perl inside your flutter project root directory. (Make sure to .gitignore it)
Then in the file, copy and paste the content this https://raw.githubusercontent.com/valbok/lcov/master/genhtml.perl
Finally open git bash and run $ ./genhtml.perl ./coverage/lcov.info -o coverage/html .
Check You are done html files at coverage/html.
On android studio, select index.html then CTRL+SHIFT+C to copy the file path.
Open Chrome and on the url bar add file:///+CTRL+V. Tap enter. You are done.
PS : Who so ever is facing an error like No common filename prefix found! or some other issue, they can try replacing content of genhtml.perl file from below link.
https://raw.githubusercontent.com/linux-test-project/lcov/master/bin/genhtml