Allow target to specify LOCAL_PREFER_INTEGRITY

The option will:
 - Produce APK with uncompressed dex
 - Declare to prefer integrity in manifest

Test: unzip -vl, dex compression looks correct with the option provided
      or not.
Test: similarly, aapt dump correct attribute
Bug: None

Change-Id: I16b9a37255150c2ad84af84087dfabb536a3b07a
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index a036267..6fe9d02 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -213,6 +213,7 @@
 LOCAL_PREBUILT_OBJ_FILES:=
 LOCAL_PREBUILT_STATIC_JAVA_LIBRARIES:=
 LOCAL_PREBUILT_STRIP_COMMENTS:=
+LOCAL_PREFER_INTEGRITY:=
 LOCAL_PRESUBMIT_DISABLED:=
 LOCAL_PRIVATE_PLATFORM_APIS:=
 LOCAL_PRIVILEGED_MODULE:=