Conditionally skip dex-preopting prebuilts.

To skip dex-preopting prebuilts (to save system image space),
set "DONT_DEXPREOPT_PREBUILTS := true".

Bug: 17772057

Change-Id: I13f10e2a9c251366f29606158f8c2fb54f8ee8b1
diff --git a/core/prebuilt_internal.mk b/core/prebuilt_internal.mk
index 5666cfd..284884c 100644
--- a/core/prebuilt_internal.mk
+++ b/core/prebuilt_internal.mk
@@ -157,8 +157,10 @@
   $(built_module) : PRIVATE_CERTIFICATE := $(LOCAL_CERTIFICATE).x509.pem
 endif
 
-# Disable dex-preopt of prebuilts to save space
+# Disable dex-preopt of prebuilts to save space, if requested.
+ifeq ($(DONT_DEXPREOPT_PREBUILTS),true)
 LOCAL_DEX_PREOPT := false
+endif
 
 #######################################
 # defines built_odex along with rule to install odex