[Settings] keys with @TestApi need to have @Readable

This allows them to be accessed by test apps.

BUG: 183223092
Test: atest CtsAppSecurityHostTestCases:android.appsecurity.cts.ReadableSettingsFieldsTest#testSecurePublicSettingsKeysAreReadable
Change-Id: I2bdcab2d9279f5ef944b17896c7b31b84ca8dc81
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 719c383..61fbef5 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -9757,6 +9757,7 @@
          * @hide
          */
         @TestApi
+        @Readable
         public static final String ACCESSIBILITY_MAGNIFICATION_CAPABILITY =
                 "accessibility_magnification_capability";