Only use a fixed timestamp when packaging.

[1] changed to use a fixed timestamp for all the generated images
(either the one under $ANDROID_PRODUCT_OUT/ or the one added into the
target_files zip). It makes 'adb sync' after 'fastboot flashall' to
resync all the files. This CL changes to use a fixed timestamp only when
packaging into the target_files zip.

[1]: commit 052ae3542be93d30108e742c5c21d5c45d9b7cf8

Bug: 24377993
Bug: 24536411
Change-Id: I209318966109fd4902468dfe96caf777464f9fe1
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index bc2667d..6bdb9d1 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -28,6 +28,7 @@
   print >> sys.stderr, "Python 2.7 or newer is required."
   sys.exit(1)
 
+import datetime
 import errno
 import os
 import shutil
@@ -120,6 +121,12 @@
   if fstab:
     image_props["fs_type"] = fstab["/" + what].fs_type
 
+  # Use a fixed timestamp (01/01/2009) when packaging the image.
+  # Bug: 24377993
+  epoch = datetime.datetime.fromtimestamp(0)
+  timestamp = (datetime.datetime(2009, 1, 1) - epoch).total_seconds()
+  image_props["timestamp"] = int(timestamp)
+
   if what == "system":
     fs_config_prefix = ""
   else:
@@ -166,6 +173,12 @@
 
   print "creating userdata.img..."
 
+  # Use a fixed timestamp (01/01/2009) when packaging the image.
+  # Bug: 24377993
+  epoch = datetime.datetime.fromtimestamp(0)
+  timestamp = (datetime.datetime(2009, 1, 1) - epoch).total_seconds()
+  image_props["timestamp"] = int(timestamp)
+
   # The name of the directory it is making an image out of matters to
   # mkyaffs2image.  So we create a temp dir, and within it we create an
   # empty dir named "data", or a symlink to the DATA dir,
@@ -210,6 +223,12 @@
 
   print "creating cache.img..."
 
+  # Use a fixed timestamp (01/01/2009) when packaging the image.
+  # Bug: 24377993
+  epoch = datetime.datetime.fromtimestamp(0)
+  timestamp = (datetime.datetime(2009, 1, 1) - epoch).total_seconds()
+  image_props["timestamp"] = int(timestamp)
+
   # The name of the directory it is making an image out of matters to
   # mkyaffs2image.  So we create a temp dir, and within it we create an
   # empty dir named "cache", and build the image from that.