Don't regenerate userdata.img when signing

The userdata.img and cache.img entries are not useful in signed builds;
because fastboot doesn't look at these two entries in the *img.zip when
flashing a device. And they aren't used elsewhere. Therefore, skip
building the image files for them when signing the target files with
sign_target_files_apks. Also, add an option "--is_signing" to avoid
adding these two images when we call add_img_to_target_files.

Change-Id: I39ba91a86d9a856d7d01771f6d1403dbf21f2011
Test: Run sign_target_files_apks on a target file and userdata/cache.img doesn't not generate.
Bug: 30642470
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 0f20f82..6d68be1 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -713,7 +713,9 @@
   common.ZipClose(input_zip)
   common.ZipClose(output_zip)
 
-  add_img_to_target_files.AddImagesToTargetFiles(args[1])
+  # Skip building userdata.img and cache.img when signing the target files.
+  new_args = ["--is_signing", args[1]]
+  add_img_to_target_files.main(new_args)
 
   print "done."