Enable preferred audio input device selection on Android

Add PreferredDevice property to BaseOptions for Android, allowing
consumers to select a specific audio input device (e.g. Bluetooth
headset, USB microphone) via AudioDeviceInfo.

Changes:
- Add BaseOptions.android.cs with PreferredDevice property typed as
  AudioDeviceInfo from Android.Media
- Apply SetPreferredDevice on both AudioRecord and MediaRecorder after
  creation, before starting recording
- Guarded with OperatingSystem.IsAndroidVersionAtLeast(23) since the
  API requires Android Marshmallow (API 23+)

Usage:
  var audioManager = (AudioManager)context.GetSystemService(Context.AudioService);
  var btDevice = audioManager.GetDevices(GetDevicesTargets.Inputs)
      .FirstOrDefault(d => d.Type == AudioDeviceType.BluetoothSco);
  var recorder = audioManager.CreateRecorder(new AudioRecorderOptions
  {
      PreferredDevice = btDevice
  });

Companion to #208 which adds the equivalent iOS/macCatalyst support.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jfversluis

src/Plugin.Maui.Audio/AudioRecorder/AudioRecorder.android.cs

@@ -107,6 +107,12 @@ public Task StartAsync(string filePath, AudioRecorderOptions? recordingOptions =
 			}
 
 			audioRecord = new AudioRecord(AudioSource.Mic, sampleRate, channelIn, encoding, bufferSize);
+
+			if (OperatingSystem.IsAndroidVersionAtLeast(23) && audioRecorderOptions.PreferredDevice is not null)
+			{
+				audioRecord.SetPreferredDevice(audioRecorderOptions.PreferredDevice);
+			}
+
 			audioRecord.StartRecording();
 			Task.Run(WriteAudioDataToFile);
 		}
@@ -137,6 +143,12 @@ public Task StartAsync(string filePath, AudioRecorderOptions? recordingOptions =
 			mediaRecorder.SetAudioEncodingBitRate(bitRate);
 			mediaRecorder.SetOutputFile(audioFilePath);
 			mediaRecorder.Prepare();
+
+			if (OperatingSystem.IsAndroidVersionAtLeast(23) && audioRecorderOptions.PreferredDevice is not null)
+			{
+				mediaRecorder.SetPreferredDevice(audioRecorderOptions.PreferredDevice);
+			}
+
 			mediaRecorder.Start();
 
 			// Set MediaRecorder "is recording" flag true

src/Plugin.Maui.Audio/BaseOptions.android.cs

@@ -0,0 +1,22 @@
+using Android.Media;
+
+namespace Plugin.Maui.Audio;
+
+partial class BaseOptions
+{
+    /// <summary>
+    /// Gets or sets the preferred audio input device for recording.
+    /// <para>
+    /// Set this to an <see cref="AudioDeviceInfo"/> obtained from
+    /// <see cref="AudioManager.GetDevices(GetDevicesTargets)"/> with
+    /// <see cref="GetDevicesTargets.Inputs"/> to record from a specific device
+    /// (e.g. a Bluetooth headset or USB microphone). When <see langword="null"/>,
+    /// the system default input device is used.
+    /// </para>
+    /// <para>
+    /// Requires Android API 23 (Marshmallow) or higher. On older API levels
+    /// this property is ignored.
+    /// </para>
+    /// </summary>
+    public AudioDeviceInfo? PreferredDevice { get; set; }
+}