Check whether the external storage is writable.

LeakCanary.java

@Override
public boolean isLeakStorageWritable() {
  if (!hasStoragePermission()) {
    return false;
  }
  String state = Environment.getExternalStorageState();
  return Environment.MEDIA_MOUNTED.equals(state);
}

@TargetApi(M)
private boolean hasStoragePermission() {
  if (SDK_INT < M) {
    return true;
  }
  return context.checkSelfPermission(WRITE_EXTERNAL_STORAGE) == PERMISSION_GRANTED;
}