要将PCM和MP3合并为AAC，可以使用Android的MediaCodec API来实现。以下是一个示例代码：

import android.media.MediaCodec;
import android.media.MediaCodecInfo;
import android.media.MediaFormat;
import android.media.MediaMuxer;
import android.media.MediaCodec.BufferInfo;
import android.util.Log;

import java.io.IOException;
import java.nio.ByteBuffer;

public class AudioEncoder {

    private static final String TAG = "AudioEncoder";
    private static final String MIME_TYPE = "audio/mp4a-latm";
    private static final int SAMPLE_RATE = 44100;
    private static final int BIT_RATE = 128000;
    private static final int CHANNEL_COUNT = 2;
    private static final int AAC_PROFILE = MediaCodecInfo.CodecProfileLevel.AACObjectLC;

    private MediaCodec mediaCodec;
    private MediaMuxer mediaMuxer;
    private int audioTrackIndex;
    private boolean isMuxerStarted;

    public void mergePCMAndMP3ToAAC(String pcmFilePath, String mp3FilePath, String aacFilePath) {
        try {
            mediaMuxer = new MediaMuxer(aacFilePath, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);
            mediaCodec = MediaCodec.createEncoderByType(MIME_TYPE);
            MediaFormat mediaFormat = MediaFormat.createAudioFormat(MIME_TYPE, SAMPLE_RATE, CHANNEL_COUNT);
            mediaFormat.setInteger(MediaFormat.KEY_BIT_RATE, BIT_RATE);
            mediaFormat.setInteger(MediaFormat.KEY_AAC_PROFILE, AAC_PROFILE);
            mediaCodec.configure(mediaFormat, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
            mediaCodec.start();

            ByteBuffer[] inputBuffers = mediaCodec.getInputBuffers();
            ByteBuffer[] outputBuffers = mediaCodec.getOutputBuffers();
            BufferInfo bufferInfo = new BufferInfo();
            int inputBufferIndex;

            // 处理PCM文件
            byte[] pcmData = getPCMData(pcmFilePath);
            inputBufferIndex = mediaCodec.dequeueInputBuffer(-1);
            ByteBuffer inputBuffer = inputBuffers[inputBufferIndex];
            inputBuffer.clear();
            inputBuffer.put(pcmData);
            mediaCodec.queueInputBuffer(inputBufferIndex, 0, pcmData.length, 0, 0);

            // 处理MP3文件
            byte[] mp3Data = getMP3Data(mp3FilePath);
            inputBufferIndex = mediaCodec.dequeueInputBuffer(-1);
            inputBuffer = inputBuffers[inputBufferIndex];
            inputBuffer.clear();
            inputBuffer.put(mp3Data);
            mediaCodec.queueInputBuffer(inputBufferIndex, 0, mp3Data.length, 0, 0);

            int outputBufferIndex;
            ByteBuffer outputBuffer;
            byte[] aacData;

            // 处理编码后的数据
            while (true) {
                outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo, 0);
                if (outputBufferIndex >= 0) {
                    outputBuffer = outputBuffers[outputBufferIndex];
                    aacData = new byte[bufferInfo.size];
                    outputBuffer.get(aacData);
                    outputBuffer.clear();
                    mediaMuxer.writeSampleData(audioTrackIndex, outputBuffer, bufferInfo);
                    mediaCodec.releaseOutputBuffer(outputBufferIndex, false);
                } else if (outputBufferIndex == MediaCodec.INFO_OUTPUT_FORMAT_CHANGED) {
                    MediaFormat format = mediaCodec.getOutputFormat();
                    audioTrackIndex = mediaMuxer.addTrack(format);
                    mediaMuxer.start();
                    isMuxerStarted = true;
                } else if (outputBufferIndex == MediaCodec.INFO_TRY_AGAIN_LATER) {
                    break;
                }
            }

            mediaCodec.stop();
            mediaCodec.release();

            if (isMuxerStarted) {
                mediaMuxer.stop();
                mediaMuxer.release();
            }
        } catch (IOException e) {
            Log.e(TAG, "mergePCMAndMP3ToAAC: " + e.getMessage());
        }
    }

    private byte[] getPCMData(String pcmFilePath) {
        // 从PCM文件中读取数据并返回byte数组
        return null;
    }

    private byte[] getMP3Data(String mp3FilePath) {
        // 从MP3文件中读取数据并返回byte数组
        return null;
    }
}

这是一个简单的示例，你需要根据你的具体需求来实现getPCMData和getMP3Data方法，这些方法用于从PCM文件和MP3文件中读取数据。

此代码会将PCM和MP3文件合并为AAC文件，并保存在指定的文件路径中。你可以使用mergePCMAndMP3ToAAC方法，传入PCM文件路径