Rename Soong variable `FlattenApex` as `Flatten_apex`.

This is so that it can be used as a Soong/Blueprint property.

Test: m
Bug: 139277987
Change-Id: I009817e040994738814c92ee007aec9ef2cfa838
diff --git a/core/soong_config.mk b/core/soong_config.mk
index d60cad0..bcd025b 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -164,7 +164,7 @@
 $(call add_json_list, BoardPlatPrivateSepolicyDirs,      $(BOARD_PLAT_PRIVATE_SEPOLICY_DIR))
 $(call add_json_list, BoardSepolicyM4Defs,               $(BOARD_SEPOLICY_M4DEFS))
 
-$(call add_json_bool, FlattenApex,                       $(filter true,$(TARGET_FLATTEN_APEX)))
+$(call add_json_bool, Flatten_apex,                      $(filter true,$(TARGET_FLATTEN_APEX)))
 
 $(call add_json_str,  DexpreoptGlobalConfig,             $(DEX_PREOPT_CONFIG))