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/clear_vars.mk b/core/clear_vars.mk
index 643a138..d9f3372 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -109,7 +109,7 @@
 LOCAL_RENDERSCRIPT_CC:=
 LOCAL_RENDERSCRIPT_TARGET_API:=
 LOCAL_BUILD_HOST_DEX:=
-LOCAL_DEX_PREOPT:=
+LOCAL_DEX_PREOPT:= # '',true,false,nostripping
 LOCAL_PROTOC_OPTIMIZE_TYPE:= # lite(default),micro,full
 LOCAL_PROTOC_FLAGS:=
 LOCAL_NO_CRT:=