Fix text missing in performance overlay (on Android)

[kjlubick]

The Android fontmgr does not return a default typeface when a nullptr style is passed in [1] (I'm not sure if this is a feature or a bug). However, calling legacyMakeTypeface will use a default sans-serif font. This adds in that backup behavior.

[1] https://github.com/google/skia/blob/4170fba2d89fc0319aad5702a15acdee4470f606/src/ports/SkFontMgr_android.cpp#L310-L312

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[flutter-dashboard]

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[flar]

I'm doing builds to verify. Hope to have it tested in the next 10-15 minutes.

[flar]

It would be nice to have an automated test for this. If we can't find a way to come up with one then we need to document why and ask for a test exemption in Discord.

[kjlubick]

Could we add a golden image test? (I'm not super sure how to do that in Flutter). @Piinks probably knows or can point us at the right person.

[Piinks]

can point us at the right person.

Absolutely! :) I am not familiar with engine goldens, but @mdebbar is!

[flar]

I have done manual verification of your branch locally and the text renders on MacOS, iOS and Android with the fix.

[chinmaygarde]

@flar Mentions that this would fail by tripping the DCHECK in unopt modes on Android. But I am not sure if there such a test harness for it. Triagers couldn't find a good testing strategy. I am fine landing as-is with the instructions in the bots message.

[matanlurey]

I am happy to take over writing a test, it was blocked on infra that should now be available thanks to @dnfield.

Filed: flutter/flutter#140182.