Don't include the dev-key on IOT products.

This patch updates the logic to use the PRODUCT_IOT variable instead of
BRILLO.

Bug: 36702887
Test: `make`; Image doesn't have the dev key.
Change-Id: I1751e97d8cdfeba83c7e4720a017a5f4dcfd49da

tools/signapk/Android.mk

@@ -30,7 +30,6 @@ LOCAL_REQUIRED_MODULES := libconscrypt_openjdk_jni
 include $(BUILD_HOST_JAVA_LIBRARY)
 
 ifeq ($(TARGET_BUILD_APPS),)
-ifeq ($(BRILLO),)
 # The post-build signing tools need signapk.jar and its shared libraries,
 # but we don't need this if we're just doing unbundled apps.
 my_dist_files := $(LOCAL_INSTALLED_MODULE) \
@@ -39,4 +38,3 @@ my_dist_files := $(LOCAL_INSTALLED_MODULE) \
 $(call dist-for-goals,droidcore,$(my_dist_files))
 my_dist_files :=
 endif
-endif