Add zip2zip tool to copy zip entries from one file to another

This doesn't do any decompression / recompression, but just copies over
the already compressed contents. So it's similar to zip -U, but allows
rewriting of the paths.

The first expected usecase is to replace img_from_target_files during
the build, since it does the equivalent of this:

zip2zip -i <target-files.zip> -o <img.zip> OTA/android-info.txt:android-info.txt IMAGES/*:.

Except it decompresses and recompresses the images, which takes over a
minute instead of a few seconds.

Change-Id: I88d0df188635088783223873f78e193272dbdf1c

third_party/zip/android.go

@@ -0,0 +1,70 @@
+// Copyright 2016 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package zip
+
+import (
+	"io"
+)
+
+func (w *Writer) CopyFrom(orig *File, newName string) error {
+	if w.last != nil && !w.last.closed {
+		if err := w.last.close(); err != nil {
+			return err
+		}
+		w.last = nil
+	}
+
+	fileHeader := orig.FileHeader
+	fileHeader.Name = newName
+	fh := &fileHeader
+	fh.Flags |= 0x8
+
+	h := &header{
+		FileHeader: fh,
+		offset:     uint64(w.cw.count),
+	}
+	w.dir = append(w.dir, h)
+
+	if err := writeHeader(w.cw, fh); err != nil {
+		return err
+	}
+
+	// Copy data
+	dataOffset, err := orig.DataOffset()
+	if err != nil {
+		return err
+	}
+	io.Copy(w.cw, io.NewSectionReader(orig.zipr, dataOffset, int64(orig.CompressedSize64)))
+
+	// Write data descriptor.
+	var buf []byte
+	if fh.isZip64() {
+		buf = make([]byte, dataDescriptor64Len)
+	} else {
+		buf = make([]byte, dataDescriptorLen)
+	}
+	b := writeBuf(buf)
+	b.uint32(dataDescriptorSignature)
+	b.uint32(fh.CRC32)
+	if fh.isZip64() {
+		b.uint64(fh.CompressedSize64)
+		b.uint64(fh.UncompressedSize64)
+	} else {
+		b.uint32(fh.CompressedSize)
+		b.uint32(fh.UncompressedSize)
+	}
+	_, err = w.cw.Write(buf)
+	return err
+}