assmeble_vintf: IGNORE_TARGET_FCM_VERSION -> VINTF_*

All "private" environment variables to assemble_vintf
(inserted by the build rules, not build system variables)
should have this prefix (to distinguish against,
e.g. PRODUCT_ENFORCE_VINTF_MANIFEST)

Test: m framework_compatibility_matrix -j
Test: libvintf_test
Bug: 65028233
Change-Id: Iba44bd6d5d9a62b61666bcf5db672482d6f36488
diff --git a/AssembleVintf.cpp b/AssembleVintf.cpp
index 7f86b94..c478928 100644
--- a/AssembleVintf.cpp
+++ b/AssembleVintf.cpp
@@ -369,7 +369,7 @@
 
     bool setDeviceFcmVersion(HalManifest* manifest) {
         // Not needed for generating empty manifest for DEVICE_FRAMEWORK_COMPATIBILITY_MATRIX_FILE.
-        if (getBooleanFlag("IGNORE_TARGET_FCM_VERSION")) {
+        if (getBooleanFlag("VINTF_IGNORE_TARGET_FCM_VERSION")) {
             return true;
         }