Fix copyFile on Android. (#392)

Author: roman-r-m

android/src/main/java/io/github/elyx0/reactnativedocumentpicker/DocumentPickerModule.java

@@ -265,8 +265,9 @@ public static String copyFile(Context context, Uri uri, File destFile) throws IO
 				if (in != null) {
 					out = new FileOutputStream(destFile);
 					byte[] buffer = new byte[1024];
-					while (in.read(buffer) > 0) {
-						out.write(buffer);
+					int len;
+					while ((len = in.read(buffer)) > 0) {
+						out.write(buffer, 0, len);
 					}
 					out.close();
 					in.close();