framework-nfc: add aconfig_declarations block

Add missing aconfig_declarations block to the framework-nfc
java_sdk_library soong module. Without this, soong doesn't know which
flags to tell metalava to retain, and metalava will effectively revert
all API changes in framework-nfc, even if the @FlaggedApi flag is
enabled.

Bug: 328923659
Test: m sdk dist && grep -e isObserveModeEnabled out/dist/data/api-versions.xml
Change-Id: Iafa82071b613f9fdd590b605ab0b07cd587ecd0f
diff --git a/nfc/Android.bp b/nfc/Android.bp
index 7dd16ba..7698e2b 100644
--- a/nfc/Android.bp
+++ b/nfc/Android.bp
@@ -76,6 +76,9 @@
         "//apex_available:platform",
         "com.android.nfcservices",
     ],
+    aconfig_declarations: [
+        "android.nfc.flags-aconfig",
+    ],
 }
 
 filegroup {