soundd: improve handling of silent periods by using fill(0) Instead of array multiplication

[deanlee]

Returning np.zeros(frames, dtype=np.float32) * self.current_volume when no sound needs to be played is a simple way to ensure silence. but introduces unnecessary CPU overhead and memory allocation:

1. Performance Impact: Creating a new numpy array and multiplying it by self.current_volume every time the callback is called can introduce unnecessary CPU overhead, especially if the callback is invoked frequently.
2. Memory Allocation: Continuously allocating memory for zeros could lead to inefficient memory usage over time.

This PR improves performance by handling silent periods more efficiently:

1. Efficient Silent Period Handling: Replaces array creation and multiplication with direct buffer filling using fill(0), eliminating redundant operations.
2. Reduced CPU and Memory Usage: Minimizes unnecessary computations and memory allocations, potentially saving up to 1% in CPU usage during silent periods based on preliminary tests.

Note::
To further optimize, consider using stream.stop() and stream.start() to effectively pause and resume the audio stream dynamically based on the current_alert state.

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UI Screenshots

[adeebshihadeh]

The current handling is intentional to keep resource usage consistent.

Ideally, every second openpilot runs is identical to every other second in terms of CPU, memory, etc.

[deanlee]

I understand. I've removed the note section for further optimization.

The CPU usage of the original code differs between silent and play modes, increasing during playback. This PR aims to reduce unnecessary resource usage in silent mode without changing the original structure, which should not violate the principle of maintaining consistent resource usage. What do you think?

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