flutter trace --stop no longer works on Android

[eseidelGoogle]

Broke as of flutter/engine@09972ad

I haven't tested on iOS, may be broken there too.
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/android/android_device.dart#L385
is the line with the bad assumption in flutter trace --stop

I suspect we should update it to pull the trace over observatory instead of via a test file? @johnmccutchan or @chinmaygarde would know?

FYI @jason-simmons

[eseidelGoogle]

I ran into this while trying to use https://github.com/flutter/flutter/blob/master/dev/profile_startup.dart

[jason-simmons]

This should be fixed now

[github-actions]

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