How avoid automatic gain control with AudioRecord?

Short answer is "Nothing".

The AudioSources correspond to various logical audio input devices depending on the accessories that you have connected to the phone and the current use-case, which in turn corresponds to physical devices (primary built-in mic, secondary mic, wired headset mic, etc) with different tunings.

Each such combination of physical device and tuning is trimmed by the OEM to meet both external requirements (e.g. CTS, operator requirements, etc) and internal acoustic requirements set by the OEM itself. This process may cause the introduction of various filters - such as AGC, noise suppression, equalization, etc - into the audio input path at the hardware codec or multimedia DSP level.

While a PURE_MIC source might be useful in for some applications, it's not something that's available today.
On many devices you can control things like microphone gain, and possibly even the filter chain, by using amixer to write to the hardware codec's ALSA controls. However, this would obviously be a very platform-specific approach, and I also suspect that you have to be running as either the root or audio user to be allowed to do this.

Note: Most of the audio sources (including DEFAULT) apply processing to the audio signal. To record raw audio select UNPROCESSED. Some devices do not support unprocessed input. Call AudioManager.getProperty("PROPERTY_SUPPORT_AUDIO_SOURCE_UNPROCESSED") first to verify it's available. If it is not, try using VOICE_RECOGNITION instead, which does not employ AGC or noise suppression. You can use UNPROCESSED as an audio source even when the property is not supported, but there is no guarantee whether the signal will be unprocessed or not in that case.

Android documentation Link https://developer.android.com/guide/topics/media/mediarecorder.html#example

    AudioManager audioManager = (AudioManager)getSystemService(Context.AUDIO_SERVICE);
    if(audioManager.getProperty(AudioManager.PROPERTY_SUPPORT_AUDIO_SOURCE_UNPROCESSED) !=null)
        mRecorder.setAudioSource(MediaRecorder.AudioSource.UNPROCESSED);
    else
        mRecorder.setAudioSource(MediaRecorder.AudioSource.VOICE_RECOGNITION);

Some devices add AGC effect to the sound input tract by default. Therefore, you need to obtain reference to corresponding AudioEffect object and force it to disable.

First, obtain AutomaticGainControl object linked to the AudioRecord audio session, and then just set it disabled:

if (AutomaticGainControl.isAvailable()) {
    AutomaticGainControl agc = AutomaticGainControl.create(
            myAudioRecord.getAudioSessionId()
        );
    agc.setEnabled(false);
}