[SettingsProvider] make nfc_payment_default_component readable

App processes might need to read the Settings value using system code.
Therefore this key is @hide but should be @Readable.

BUG: 191519696
Test: atest android.appsecurity.cts.ReadableSettingsFieldsTest
Change-Id: Idf1512ebf3487d74a18ad2dc13ee7cbb2c7643d0

core/java/android/provider/Settings.java

@@ -8748,6 +8748,7 @@ public final class Settings {
          */
         @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
         @TestApi
+        @Readable
         public static final String NFC_PAYMENT_DEFAULT_COMPONENT = "nfc_payment_default_component";
 
         /**