Restrict usage of AccessibilityManager::mIsEnabled after P

Setting this field with reflection leaves AccessibilityManager in
improper state. Restrict access to this field in future apis

Bug: b/124052733
Test: accessibility tests passed
Change-Id: I776e1092ab33cf9660f593a22585695c17852e1e
diff --git a/core/java/android/view/accessibility/AccessibilityManager.java b/core/java/android/view/accessibility/AccessibilityManager.java
index 06207a9..384cdbb 100644
--- a/core/java/android/view/accessibility/AccessibilityManager.java
+++ b/core/java/android/view/accessibility/AccessibilityManager.java
@@ -174,7 +174,7 @@
 
     final Handler.Callback mCallback;
 
-    @UnsupportedAppUsage
+    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
     boolean mIsEnabled;
 
     int mRelevantEventTypes = AccessibilityEvent.TYPES_ALL_MASK;