Convert deflate image chunks to raw if the raw data is smaller

The imgpatch will fail on empty deflates because the bspatch won't call
the sink function if the target length is zero.

Instead of compressing an empty string, it's cleaner to not generate such
empty deflate chunks in the patch. Therefore, we can just convert the
chunk type to raw if the target length is smaller than the patch data.

Also adjust some unit tests and add the testdata gzipped_source &
gzipped_target. These two files are ~1K each and are generated by
gzipping two slightly different regular files.

Bug: 79265132
Test: unit tests pass, imgpatch applys successfully on the given src/tgt
Change-Id: I6bfff3251918137f6762a6f9e9551642371a1124

applypatch/include/applypatch/imgdiff_image.h

@@ -44,6 +44,8 @@ class ImageChunk {
   int GetType() const {
     return type_;
   }
+
+  const uint8_t* GetRawData() const;
   size_t GetRawDataLength() const {
     return raw_data_len_;
   }
@@ -99,7 +101,6 @@ class ImageChunk {
                         bsdiff::SuffixArrayIndexInterface** bsdiff_cache);
 
  private:
-  const uint8_t* GetRawData() const;
   bool TryReconstruction(int level);
 
   int type_;                                    // CHUNK_NORMAL, CHUNK_DEFLATE, CHUNK_RAW