ExifInterface: Write new XMP data out to a separate segment

The XMP spec part 3 section 3.3.2 makes it clear that JPEG files should
store XMP data in separate APP1 segment, not the Exif segment under tag
700. ExifInterface is documented to (incorrectly) prefer XMP data stored
in the Exif segment if both are present. But the behaviour of where XMP
data is added in the file if it wasn't previously present isn't
explicitly documented. This change ensures that XMP data added to a file
that didn't previously have any is added in a standalone segment.

This change also adds some additional documentation to describe this
behaviour.

Test: ExifInterfaceTest
Change-Id: I84328802a3b43b4766850fcdc6731c1e7c59e909

app/src/main/java/io/github/tommygeenexus/exifinterfaceextended/ExifInterfaceExtended.java

@@ -5565,7 +5565,13 @@ private void getJpegAttributes(final ByteOrderedDataInputStream source,
                                 + IDENTIFIER_EXIF_APP1.length;
                         readExifSegment(value, imageType);
                         setThumbnailData(new ByteOrderedDataInputStream(value));
-                        if (xmpBeforeReadingExif != getAttributeBytes(TAG_XMP)) {
+                        if (getAttributeBytes(TAG_XMP) == null) {
+                            // XMP should be stored in a separate APP1 segment (see XMP spec part 3
+                            // section 3.3.2). If the Exif segment didn't contain XMP then we set
+                            // this to true to ensure any XMP data added will get written out to a
+                            // separate segment.
+                            mXmpIsFromSeparateMarker = true;
+                        } else if (xmpBeforeReadingExif != getAttributeBytes(TAG_XMP)) {
                             mXmpIsFromSeparateMarker = false;
                         }
                     } else if (ExifInterfaceExtendedUtils.startsWith(bytes, IDENTIFIER_XMP_APP1)) {