Annotate android LocalCache.spliterator() with @IgnoreJRERequirement.

CI's animal-sniffer check on the android variant (gummy-bears-api-23) rejected
the Spliterator override added in the previous commit because java.util.Spliterator
and Spliterators.spliteratorUnknownSize(...) are not part of Android API 23.

Following the established pattern in ImmutableCollection.java, Ints.java, and
Longs.java under android/guava/src, annotate the method with the package-local
@IgnoreJRERequirement.  The method is only invoked from APIs whose signatures
already carry Java 8 types, so callers are already gated by the same constraint.

The guava/src variant is unaffected because it uses the java18 signature, which
includes Spliterator.

Made-with: Cursor

Author: holly-agyei

android/guava/src/com/google/common/cache/LocalCache.java

@@ -4406,6 +4406,7 @@ public void clear() {
     }
 
     @Override
+    @IgnoreJRERequirement // used only from APIs with Java 8 types in them
     public Spliterator<T> spliterator() {
       return Spliterators.spliteratorUnknownSize(
           iterator(), Spliterator.CONCURRENT | Spliterator.DISTINCT | Spliterator.NONNULL);