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: I8840811fde92a71e97ed100aacfc7fbb003580d0
diff --git a/android/variable.go b/android/variable.go
index 8886bae..0931db8 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -277,7 +277,7 @@
 	Ndk_abis               *bool `json:",omitempty"`
 	Exclude_draft_ndk_apis *bool `json:",omitempty"`
 
-	FlattenApex *bool `json:",omitempty"`
+	Flatten_apex *bool `json:",omitempty"`
 
 	DexpreoptGlobalConfig *string `json:",omitempty"`