Set the container field of aconfig flags

Please see go/aconfig-flag-containers for more context

Test: m
Bug: 312769710
Change-Id: Ibe1f32691a2fb10a267b580766abcde621a596ff
diff --git a/services/surfaceflinger/surfaceflinger_flags.aconfig b/services/surfaceflinger/surfaceflinger_flags.aconfig
index c723a0f..7202c9e 100644
--- a/services/surfaceflinger/surfaceflinger_flags.aconfig
+++ b/services/surfaceflinger/surfaceflinger_flags.aconfig
@@ -1,4 +1,5 @@
 package: "com.android.graphics.surfaceflinger.flags"
+container: "system"
 
 flag {
   name: "misc1"