`flutter pub global run devtools` blocks other `flutter` commands

[liyuqian]

I was following https://docs.flutter.dev/tools/devtools/cli and flutter pub global run devtools blocked flutter run --observatory-port=9200 from running. Maybe we should remove flutter pub global run devtools from the instruction and only suggest pub global run devtools.

[liyuqian]

CC @kenzieschmoll

[devoncarew]

I think that some portion of the flutter pub commands should release the flutter tools lock after they start. Perhaps all of them? Otherwise long running tasks, like flutter pub global run, will block other invocations of the tool.

cc @jonahwilliams