Fix bind_keyguard_media_visibility

Purpose was deleted during rebase. This CL puts it back.
Fixes: 327542570
Flag: None
Test: presubmit

Change-Id: I3362b1e1c17962d129f68cd491e764083cc13007
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index cb6894e..0ba57ee 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -542,6 +542,9 @@
     namespace: "systemui"
     description: "Binds Keyguard Media Controller Visibility to MediaContainerView"
     bug: "298213983"
+    metadata {
+        purpose: PURPOSE_BUGFIX
+    }
 }
 
 flag {