allow prebuilts to specify LOCAL_CERTIFICATE so they can be resigned

If a prebuilt specifies a LOCAL_CERTIFICATE, stick it in the
PACKAGES.* collection so that the predexopt process can work on the
file (which only works if the new .apk can be signed).
diff --git a/core/ b/core/
index 34d2453..4cbbb99 100644
--- a/core/
+++ b/core/
@@ -40,6 +40,21 @@
+  # can't re-sign this package, so predexopt is not available.
+# If this is not an absolute certificate, assign it to a generic one.
+ifeq ($(dir $(strip $(LOCAL_CERTIFICATE))),./)
 ifneq ($(prebuilt_module_is_a_library),)
   ifneq ($(LOCAL_IS_HOST_MODULE),)