Launch DevTools from "dart devtools" instead of "pub global" when using SDK >= 2.15

[DanTup]

See flutter/flutter#90894 (comment).

We can skip activating via Pub, checking for updates, etc. if we use the SDK-supplied command.

From flutter/flutter#90894 (comment)

2.15.0-39.0.dev is the earliest dart version this commit was included in.

@helin24 currently for internal workspaces we set startDevToolsFromDaemon and start from the Flutter daemon. I'm assuming that should remain as-is, but if that's no longer required let me know and I can remove it.

[helin24]

@kenzieschmoll I believe we can use dart directly for internal workspaces, is that correct? Internal workspace's have a dart with a version above 2.15.0-39.0.dev and can therefore start devtools the same way? (asking now as I'm making this change for IntelliJ)

[kenzieschmoll]

there are pre-built binaries DevTools binaries in google3 that should be used to start devtools. The DevTools build bundled with the external Dart SDK is not the same as the google3 DevTools.