fix: stop busy-looping while MP3 recording is paused

[MiMoHo]

Type of change(s)

• Bug fix

What changed and why

When recording in MP3 format, Mp3Recorder runs its capture/encode loop on a background thread:

while (!isStopped.get()) {
    if (!isPaused.get()) { ...read/encode/write... }
}

While the recording is paused (pause() sets isPaused = true), the inner branch is skipped but the enclosing while (!isStopped.get()) keeps re-evaluating with no read, wait, or sleep. This is a tight busy-loop that pins one CPU core at ~100% for the entire pause duration, which is the high CPU usage / battery drain reported in the issue. The MediaRecorder-based recorder is unaffected because it uses the platform pause()/resume().

This change blocks the recording thread on a monitor (pauseLock.wait()) while paused instead of spinning, and wakes it from resume() and stop() via notifyAll(). The wait() is guarded by a re-checking while (isPaused.get() && !isStopped.get()) loop to handle spurious wakeups, and both the state change and the notify happen under pauseLock, so a wakeup cannot be lost. audioRecord is left running during pause exactly as before (it was only stopped in stop()), so no captured-audio behavior changes — the OS buffer simply overflows and is discarded while paused, as it always did.

Tests performed

• detekt, lint, unit tests and the build all pass locally (CI-equivalent).
• Source-verified: the MP3 recording thread no longer busy-loops polling isPaused; it now blocks on a pauseLock monitor (wait() / notifyAll() on resume/stop), eliminating the pause-time CPU/battery drain. Standard, contained thread-synchronization change.
• Note: verified via CI + code review; on-device CPU profiling during pause was not scripted.

Closes the following issue(s)

• Closes Busy loop when pausing MP3 recording #324

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (verified via CI + source review; see Tests performed).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.

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