Move build number file to out/soong

Move the build number file to out/soong so Soong can treat it
as an output path, and pass the name of the file to Soong instead
of the command to read it.

Test: m checkbuild
Change-Id: I9e1aecf33d11fe4fde34cd57cc9b1f1ad49b0424
Bug: 153071808
Merged-In: I9e1aecf33d11fe4fde34cd57cc9b1f1ad49b0424
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 6ea8d60..c7d16d0 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -25,7 +25,7 @@
 $(call add_json_str,  Make_suffix, -$(TARGET_PRODUCT))
 
 $(call add_json_str,  BuildId,                           $(BUILD_ID))
-$(call add_json_str,  BuildNumberFromFile,               $(BUILD_NUMBER_FROM_FILE))
+$(call add_json_str,  BuildNumberFile,                   build_number.txt)
 
 $(call add_json_str,  Platform_version_name,             $(PLATFORM_VERSION))
 $(call add_json_val,  Platform_sdk_version,              $(PLATFORM_SDK_VERSION))