new LOCAL_DEX_PREOPT option "nostripping"

Set "LOCAL_DEX_PREOPT := nostripping" to do dex-preopt while not strip
the classes.dex.

Bug: 5396625 5502338
Change-Id: Ie8dea4414ebeefeae89c6433b246faea27baa5e7
diff --git a/core/base_rules.mk b/core/base_rules.mk
index e18b6ec..242b53d 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -528,7 +528,7 @@
 	$(copy-file-to-target-with-cp)
 endif
 
-ifeq ($(LOCAL_DEX_PREOPT),true)
+ifdef LOCAL_DEX_PREOPT
 installed_odex := $(basename $(LOCAL_INSTALLED_MODULE)).odex
 built_odex := $(basename $(LOCAL_BUILT_MODULE)).odex
 $(installed_odex) : $(built_odex) | $(ACP)