make sure we always save database as KDBX4 when using ChallengeXCKey (for compatibility with KeepassXC), closes #596

src/KeePassLib2Android/Keys/IUserKey.cs

@@ -38,9 +38,11 @@ namespace KeePassLib.Keys
 			get;
 		}
 
-		// /// <summary>
-		// /// Clear the key and securely erase all security-critical information.
-		// /// </summary>
-		// void Clear();
+        // /// <summary>
+        // /// Clear the key and securely erase all security-critical information.
+        // /// </summary>
+        // void Clear();
+
+        uint GetMinKdbxVersion();
 	}
 }

src/KeePassLib2Android/Keys/KcpCustomKey.cs

@@ -45,7 +45,12 @@ namespace KeePassLib.Keys
 			get { return m_pbKey; }
 		}
 
-		public KcpCustomKey(string strName, byte[] pbKeyData, bool bPerformHash)
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
+	    public KcpCustomKey(string strName, byte[] pbKeyData, bool bPerformHash)
 		{
 			Debug.Assert(strName != null); if(strName == null) throw new ArgumentNullException("strName");
 			Debug.Assert(pbKeyData != null); if(pbKeyData == null) throw new ArgumentNullException("pbKeyData");

src/KeePassLib2Android/Keys/KcpKeyFile.cs

@@ -64,7 +64,12 @@ namespace KeePassLib.Keys
 			get { return m_pbKeyData; }
 		}
 
-		public IOConnectionInfo Ioc
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
+	    public IOConnectionInfo Ioc
 		{
 			get { return m_ioc; }
 		}

src/KeePassLib2Android/Keys/KcpPassword.cs

@@ -53,7 +53,12 @@ namespace KeePassLib.Keys
 			get { return m_pbKeyData; }
 		}
 
-		public KcpPassword(byte[] pbPasswordUtf8)
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
+	    public KcpPassword(byte[] pbPasswordUtf8)
 		{
 			SetKey(pbPasswordUtf8);
 		}

src/KeePassLib2Android/Keys/KcpUserAccount.cs

@@ -60,7 +60,12 @@ namespace KeePassLib.Keys
 			get { return m_pbKeyData; }
 		}
 
-		/// <summary>
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
+	    /// <summary>
 		/// Construct a user account key.
 		/// </summary>
 		public KcpUserAccount()