Disable com.android.runtime.host on darwin

The apex depends on binaries that are not buildable on darwin hosts.

Test: m nothing
Bug: 120634314
Change-Id: I076deaf2fed66d06da3ad000b526f6496b80ef9f
diff --git a/build/apex/Android.bp b/build/apex/Android.bp
index b3c23b3..88178a0 100644
--- a/build/apex/Android.bp
+++ b/build/apex/Android.bp
@@ -170,4 +170,9 @@
         }
     },
     key: "com.android.runtime.key",
+    target: {
+        darwin: {
+            enabled: false,
+        },
+    },
 }