AudioResampler: initialize frame_.samples_per_channel_

[Be-ing]

This change is needed for the same reason as b776e64a3e1a33b92ffe94ffba389a318ae87f41 (#722):

A change in the resampler API introduced in libwebrtc m137 caused the output frame's samples_per_channel_ property to remain uninitialized. As a result, the length of the output slice was always calculated as zero, resulting in NativeAudioStream yielding audio frames without any sample data.

[Be-ing]

I'm unclear why #722 made a change at a higher level of the stack in NativeAudioSink rather than fixing it here. Zed calls AudioResampler::remix_and_resample directly, so switching to this repository's main branch broke audio playback in Zed.

[Be-ing]

@xianshijing-lk could you take a look at this? Zed panics on livekit's main branch without this one line change.

[Be-ing]

Ping @xianshijing-lk, could you take a look at this one line fix?

[xianshijing-lk]

I just landed it. Sorry for missing the email notification.