Fix password focus in lockscreen

When navigating the listview with the keyboard, the focus of the
password field can get lost as it moves to the button. This patch
reclaims the focus whenever the password field becomes visible.

REVIEW:121253

lookandfeel/contents/lockscreen/LockScreen.qml

@@ -134,6 +134,11 @@ Image {
                             onAccepted: unlockFunction()
                             focus: true
                             visible: block.mainItem.model.get(block.mainItem.selectedIndex)["showPassword"]
+                            onVisibleChanged: {
+                                if (visible) {
+                                    forceActiveFocus();
+                                }
+                            }
 
                             Keys.onLeftPressed: {
                                 if (text == "") {