Returns the minimum index type from all targets (#3656)

We need to return the least restrictive index type and not the first one we find

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/local/IndexManager.java

@@ -33,7 +33,7 @@
  */
 public interface IndexManager {
   /** Represents the index state as it relates to a particular target. */
-  public enum IndexType {
+  enum IndexType {
     /** Indicates that no index could be found for serving the target. */
     NONE,
     /**

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLiteIndexManager.java

@@ -313,17 +313,19 @@ public IndexOffset getMinOffset(String collectionGroup) {
 
   @Override
   public IndexType getIndexType(Target target) {
+    IndexType result = IndexType.FULL;
     for (Target subTarget : getSubTargets(target)) {
       FieldIndex index = getFieldIndex(subTarget);
       if (index == null) {
-        return IndexType.NONE;
+        result = IndexType.NONE;
+        break;
       }
 
       if (index.getSegments().size() < subTarget.getSegmentCount()) {
-        return IndexType.PARTIAL;
+        result = IndexType.PARTIAL;
       }
     }
-    return IndexType.FULL;
+    return result;
   }
 
   @Override