manager: fix return value for accountCount from bool to std::size_t

This value is used in getAccountList() to reserve the size of the final
vector that will group all the account pointers.

Change-Id: Id2b0ae149b08b7962123d9e7533fe3c34e8d4750

src/manager.h

@@ -838,7 +838,7 @@ public:
     }
 
     template<class T = Account>
-    bool accountCount() const
+    std::size_t accountCount() const
     {
         return accountFactory.accountCount<T>();
     }