ICU-22761 Optimize get value of LocaleObjectCache

When invoking get(K key) method of LocaleObjectCache, it may be blocked for a long time, looping in lines 45 and 53, because after the referent of SoftReference is set to NULL, it may take a long time to add SoftReference to its ReferenceQueue if GC is slowly.

For example, in the Concurrent Copying GC in Android Art virtual machine, the referent in SoftReference is set to null during the copying phase, while the SoftReference is added to its ReferenceQueue in the ReferenceQueueDaemon thread after GC, which experiences a long time in between

icu4j/main/core/src/main/java/com/ibm/icu/impl/locale/LocaleObjectCache.java

@@ -34,24 +34,34 @@ public V get(K key) {
         }
         if (value == null) {
             key = normalizeKey(key);
-            V newVal = createObject(key);
-            if (key == null || newVal == null) {
-                // subclass must return non-null key/value object
+            // subclass must return non-null key object
+            if (key == null) {
                 return null;
             }
 
-            CacheEntry<K, V> newEntry = new CacheEntry<K, V>(key, newVal, _queue);
+            entry = _map.get(key);
+            if (entry != null) {
+                value = entry.get();
+            }
+            // hit cache
+            if (value != null) {
+                return value;
+            }
 
-            while (value == null) {
-                cleanStaleEntries();
-                entry = _map.putIfAbsent(key, newEntry);
-                if (entry == null) {
-                    value = newVal;
-                    break;
-                } else {
-                    value = entry.get();
-                }
+            // if map not contains key or the referent value of CacheEntry is set to be null
+            // both need create a new value
+            V newVal = createObject(key);
+            if (newVal == null) {
+                // subclass must return non-null value object
+                return null;
             }
+
+            CacheEntry<K, V> newEntry = new CacheEntry<K, V>(key, newVal, _queue);
+            // just replace it
+            _map.put(key, newEntry);
+            // clean recycled SoftReferences again
+            cleanStaleEntries();
+            return newVal;
         }
         return value;
     }