Hide CFI behind a global flag.

This CL ensures that locally enabling CFI in a .bp file is not honored
unless it is enabled globally using ENABLE_CFI=true first, effectively
hiding it behind a flag.

Bug: 30227045
Bug: 22033465
Test: m -j40 works and device boots
Test: cfi is correctly honored only when the global flag is set.
Change-Id: Iccc6e4bf5e7828ab8ce6056f3e40922712faa0d8
diff --git a/android/variable.go b/android/variable.go
index 271454a..bb84be2 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -124,6 +124,7 @@
 	UseGoma                    *bool `json:",omitempty"`
 	Debuggable                 *bool `json:",omitempty"`
 	Eng                        *bool `json:",omitempty"`
+	EnableCFI                  *bool `json:",omitempty"`
 
 	VendorPath *string `json:",omitempty"`