Set the container field of aconfig flags

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

Test: m
Bug: 312769710
Change-Id: I95efda5d266ad45202501d82d256d9e9f8e7826a
diff --git a/aconfig/launcher.aconfig b/aconfig/launcher.aconfig
index c13c68c..20b4834 100644
--- a/aconfig/launcher.aconfig
+++ b/aconfig/launcher.aconfig
@@ -1,4 +1,5 @@
 package: "com.android.launcher3"
+container: "system"
 
 flag {
     name: "enable_expanding_pause_work_button"