add "EXTERNAL" as special value of LOCAL_CERTIFICATE

Setting LOCAL_CERTIFICATE to "EXTERNAL" now marks an apk (either a
prebuilt or otherwise) as needing the default test key within the
system, but one that should be signed after the target_files is
produced but before sign_target_files_apks does the rest of the
signing.  (We use this to ship apps on the system that are signed by
third parties, like Facebook.)
diff --git a/core/prebuilt.mk b/core/prebuilt.mk
index 2693f5d..b03f2af 100644
--- a/core/prebuilt.mk
+++ b/core/prebuilt.mk
@@ -42,6 +42,16 @@
 endif
 endif
 
+ifeq ($(LOCAL_CERTIFICATE),EXTERNAL)
+  # The magic string "EXTERNAL" means this package will be signed with
+  # the test key throughout the build process, but we expect the final
+  # package to be signed with a different key.
+  #
+  # This can be used for packages where we don't have access to the
+  # keys, but want the package to be predexopt'ed.
+  LOCAL_CERTIFICATE := testkey
+  PACKAGES.$(LOCAL_MODULE).EXTERNAL_KEY := 1
+endif
 ifeq ($(LOCAL_CERTIFICATE),)
   ifneq ($(filter APPS,$(LOCAL_MODULE_CLASS)),)
     # It is now a build error to add a prebuilt .apk without