traymanager: improve manage/unmanaer

the selection of _NET_SYSTEM_TRAY_Sn should be destroyed when the
manager lost.

Exported three methods: Manage/Unmanage/RetryManager

RetryManager may send the signal of Added if there has any TrayIcons after
invoked Unmanager and Manager.

Change-Id: I40197944c94a86eabb12573ca8ef331d98e701cb