Refactor token state management in MeasureUnitImpl

- Introduced an enum `TokenState` to improve clarity and maintainability of token state definitions.
- Replaced integer state management with the new enum for better readability in the `nextSingleUnit` method.
- Updated conditional checks to utilize the new `TokenState` enum, enhancing code consistency.

icu4j/main/core/src/main/java/com/ibm/icu/impl/units/MeasureUnitImpl.java

@@ -612,6 +612,18 @@ private MeasureUnitImpl parse() {
             return result;
         }
 
+        /**
+         * Token states definitions.
+         */
+        enum TokenState {
+            // No tokens seen yet (will accept power, SI or binary prefix, or simple unit)
+            NO_TOKENS_SEEN,
+            // Power token seen (will not accept another power token)
+            POWER_TOKEN_SEEN,
+            // SI or binary prefix token seen (will not accept a power, or SI or binary prefix token)
+            PREFIX_TOKEN_SEEN
+        }
+
         /**
          * Returns the next "single unit" via result.
          * <p>
@@ -627,13 +639,7 @@ private MeasureUnitImpl parse() {
         private SingleUnitOrConstant nextSingleUnit() {
             SingleUnitImpl result = new SingleUnitImpl();
 
-            // State definitions:
-            // 0 = No tokens seen yet (will accept power, SI or binary prefix, or simple
-            // unit)
-            // 1 = Power token seen (will not accept another power token)
-            // 2 = SI or binary prefix token seen (will not accept a power, or SI or binary
-            // prefix token)
-            int state = 0;
+            TokenState state = TokenState.NO_TOKENS_SEEN;
 
             boolean atStart = fIndex == 0;
             Token token = nextToken();
@@ -704,21 +710,21 @@ private SingleUnitOrConstant nextSingleUnit() {
             while (true) {
                 switch (token.getType()) {
                     case TYPE_POWER_PART:
-                        if (state > 0) {
+                        if (state != TokenState.NO_TOKENS_SEEN) {
                             throw new IllegalArgumentException();
                         }
 
                         result.setDimensionality(result.getDimensionality() * token.getPower());
-                        state = 1;
+                        state = TokenState.POWER_TOKEN_SEEN;
                         break;
 
                     case TYPE_PREFIX:
-                        if (state > 1) {
+                        if (state == TokenState.PREFIX_TOKEN_SEEN) {
                             throw new IllegalArgumentException();
                         }
 
                         result.setPrefix(token.getPrefix());
-                        state = 2;
+                        state = TokenState.PREFIX_TOKEN_SEEN;
                         break;
 
                     case TYPE_SIMPLE_UNIT: