make FIRDocumentListenOptions.includeMetadataChanges private

as discussed in Firestore API discussion, the name clashes creates confusion. Android SDK already not exposing the property; here change iOS SDK to match it.

Author: zxu123

Firestore/Source/API/FIRDocumentReference.m

@@ -48,6 +48,8 @@ @interface FIRDocumentListenOptions ()
 - (instancetype)initWithIncludeMetadataChanges:(BOOL)includeMetadataChanges
     NS_DESIGNATED_INITIALIZER;
 
+@property(nonatomic, assign, readonly) BOOL includeMetadataChanges;
+
 @end
 
 @implementation FIRDocumentListenOptions

Firestore/Source/Public/FIRDocumentReference.h

@@ -36,8 +36,6 @@ NS_SWIFT_NAME(DocumentListenOptions)
 
 - (instancetype)init;
 
-@property(nonatomic, assign, readonly) BOOL includeMetadataChanges;
-
 /**
  * Sets the includeMetadataChanges option which controls whether metadata-only changes (i.e. only
  * `FIRDocumentSnapshot.metadata` changed) should trigger snapshot events. Default is NO.