Fix compiler error when build with mono 2.0

Using an unassigned Action in lambda function will cause compiler
error for older version of Mono, even the variable will be assigned
before it is used.

The workaround is to assign null to the Action first.

Change-Id: I6ecdd12caf559f7509d1a3737127fd2cc83c50e6

Author: chkuang-g

source/AndroidResolver/src/GradleResolver.cs

@@ -709,7 +709,7 @@ private void FindAndResolveConflicts(Action complete) {
             var conflictsEnumerator = conflicts.GetEnumerator();
 
             // Move to the next conflicting package and prompt the user to delete a package.
-            Action promptToDeleteNextConflict;
+            Action promptToDeleteNextConflict = null;
 
             promptToDeleteNextConflict = () => {
                 bool conflictEnumerationComplete = false;