Move the implementation of cd entry map to a separate file

Move the entry map classes to a separate file to make the hierarchy
clear.

Test: unittests pass
Change-Id: Ie01d7835359daa4f59af75a0eda204c696d5658e
diff --git a/libziparchive/Android.bp b/libziparchive/Android.bp
index 1bbffaf..3d4e86e 100644
--- a/libziparchive/Android.bp
+++ b/libziparchive/Android.bp
@@ -60,6 +60,7 @@
     srcs: [
         "zip_archive.cc",
         "zip_archive_stream_entry.cc",
+        "zip_cd_entry_map.cc",
         "zip_writer.cc",
     ],