Add SoLoaderLibrary annotation stub for Gradle builds

Summary:
Add a stub `SoLoaderLibrary` annotation at
`com.facebook.soloader.annotation.SoLoaderLibrary` for open-source Gradle
builds. The real annotation lives in `fbandroid/java/com/facebook/soloader/annotation/`
and is used by the internal Buck build for compile-time validation of native
library merging. Since it is not published to Maven Central, the Gradle build
fails with "Unresolved reference" when any ReactAndroid file uses it.

The stub follows the existing pattern used by `DebugOverlayTag` and
`ProcessorBase`: the file lives in the shared source tree and gets compiled
by Gradle, while the BUCK file has `autoglob = False` with no `srcs`, making
it an empty target so Buck continues using the real internal annotation.

Differential Revision: D94950035

Author: mdvacca

packages/react-native/ReactAndroid/src/main/java/com/facebook/soloader/annotation/SoLoaderLibrary.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.soloader.annotation
+
+/**
+ * Stub annotation for open-source Gradle builds. The real implementation lives in
+ * fbandroid/java/com/facebook/soloader/annotation/ and is used by the internal Buck build for
+ * compile-time validation of native library merging.
+ */
+@Retention(AnnotationRetention.SOURCE)
+@Target(AnnotationTarget.CLASS)
+internal annotation class SoLoaderLibrary(vararg val value: String)