feat(joystick)!: add the feature flag

Add a flag for enabling magnification joystick feature

Bug: 297211257
Bug: 257316431
Bug: 293223352
Test: build pass and abd command to flip the flag locally
Change-Id: I8b287c225ccb1b8ea648118911772394e666d14e
diff --git a/services/accessibility/accessibility.aconfig b/services/accessibility/accessibility.aconfig
index 0fc491a..3709f47 100644
--- a/services/accessibility/accessibility.aconfig
+++ b/services/accessibility/accessibility.aconfig
@@ -13,3 +13,10 @@
     description: "Whether to enable multi-finger-multi-tap gesture for magnification"
     bug: "257274411"
 }
+
+flag {
+    name: "enable_magnification_joystick"
+    namespace: "accessibility"
+    description: "Whether to enable joystick controls for magnification"
+    bug: "297211257"
+}
\ No newline at end of file