BF_BUILD_NUMBER should have BUILD_NUMBER

BF_BUILD_NUMBER has hostname by mistake during refactoring.

Bug: 286453356
Test: build with BUILD_NUMBER
Change-Id: Id3fd7975057dfd70dfb7c02650672e670c6d5984
diff --git a/core/sysprop.mk b/core/sysprop.mk
index cf4b1f6..a2296a8 100644
--- a/core/sysprop.mk
+++ b/core/sysprop.mk
@@ -174,7 +174,7 @@
   ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
     BF_BUILD_NUMBER := $(BUILD_USERNAME)$$($(DATE_FROM_FILE) +%m%d%H%M)
   else
-    BF_BUILD_NUMBER := $$(cat $(SOONG_OUT_DIR)/build_hostname.txt)
+    BF_BUILD_NUMBER := $(BUILD_NUMBER_FROM_FILE)
   endif
   BUILD_FINGERPRINT := $(PRODUCT_BRAND)/$(TARGET_PRODUCT)/$(TARGET_DEVICE):$(PLATFORM_VERSION)/$(BUILD_ID)/$(BF_BUILD_NUMBER):$(TARGET_BUILD_VARIANT)/$(BUILD_VERSION_TAGS)
 endif