[flutter_tools] According to AnalysisSeverity return exit code detailed proposal (#61589)

No matter what level(error, warning, info) issues flutter analyze always return fatal exit code(1). CI/CD environment receive 1(!0). This may leads to e.g. Jenkins Build step 'Execute shell' marked build as failure.
I propose according to AnalysisSeverity level return fatal(1) or success(0) exit code.