Move startup perf measurements to a background thread

[badboy]

Similar fix as done on Fenix: https://github.com/mozilla-mobile/fenix/pull/26165 Should avoid loading the Glean core library too early, but still records the data just fine.

Note that Focus doesn't record startup time, so nothing to delay there.

Fixes #7216

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I haven't run that locally yet, so I'm not 100% sure if it addresses it all.

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Fixes #7216

[badboy]

Ok, the test fails so I need to look into this.

[badboy]

It doesn't fail for me locally.

[mcarare]

See https://github.com/mozilla-mobile/fenix/pull/26544 for the Fenix fix for the tests.

[badboy]

Rebased and applied the same fix that Fenix did