[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

core/java/android/provider/Settings.java

@@ -9757,6 +9757,7 @@ public final class Settings {
          * @hide
          */
         @TestApi
+        @Readable
         public static final String ACCESSIBILITY_MAGNIFICATION_CAPABILITY =
                 "accessibility_magnification_capability";