sign_target_files_apks: rewrite build.prop correctly in eng builds

In eng builds, ro.display.id has many space separated items and was
resulting in an error when trying to rewrite it as 'value' gets
turned into a list and never converted back to a string.

Change-Id: I6c8633ed2eb52c56a4097992a32d53d80df4f844
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/tools/releasetools/sign_target_files_apks b/tools/releasetools/sign_target_files_apks
index 00693b8..ab24706 100755
--- a/tools/releasetools/sign_target_files_apks
+++ b/tools/releasetools/sign_target_files_apks
@@ -235,8 +235,9 @@
       elif key == "ro.build.display.id":
         # change, eg, "JWR66N dev-keys" to "JWR66N"
         value = value.split()
-        if len(value) == 2 and value[1].endswith("-keys"):
-          value = value[0]
+        if len(value) >  1 and value[-1].endswith("-keys"):
+          value.pop()
+        value = " ".join(value)
       line = key + "=" + value
     if line != original_line:
       print "  replace: ", original_line