rename BUILT_SYSTEM_{COMPATIBILITY_,}MATRIX

For consistency with BUILT_VENDOR_MATRIX.

Bug: 66917623
Test: manual
Change-Id: I666eceeb9fc20e5f5754f605c2fbb6530e51b326
diff --git a/compatibility_matrices/Android.mk b/compatibility_matrices/Android.mk
index 6b40766..6446beb 100644
--- a/compatibility_matrices/Android.mk
+++ b/compatibility_matrices/Android.mk
@@ -136,6 +136,6 @@
 endif
 
 include $(BUILD_FRAMEWORK_COMPATIBILITY_MATRIX)
-BUILT_SYSTEM_COMPATIBILITY_MATRIX := $(LOCAL_BUILT_MODULE)
+BUILT_SYSTEM_MATRIX := $(LOCAL_BUILT_MODULE)
 
 BUILD_FRAMEWORK_COMPATIBILITY_MATRIX :=