Rename Gantry flag for fullscreen magnifier + mouse bugfix

Fullscreen magnifier moves with mouse is now a bugfix instead
of a feature, so we must rename the flag.

Bug: 354696546
Test: Existing
Flag: com.server.accessibility.enable_magnification_follows_mouse
Change-Id: I3d8289396fa4e951238cf64d503ce37e5a0f9e1f
diff --git a/services/accessibility/accessibility.aconfig b/services/accessibility/accessibility.aconfig
index 034127c..95c0753 100644
--- a/services/accessibility/accessibility.aconfig
+++ b/services/accessibility/accessibility.aconfig
@@ -114,10 +114,13 @@
 }
 
 flag {
-    name: "enable_magnification_follows_mouse"
+    name: "enable_magnification_follows_mouse_bugfix"
     namespace: "accessibility"
     description: "Whether to enable mouse following for fullscreen magnification"
     bug: "354696546"
+    metadata {
+      purpose: PURPOSE_BUGFIX
+    }
 }
 
 flag {