[Tizen.System.Storage] Register eventhandler if it is already existed

[Tizen.System.Storage] Register eventhandler if it is already existed

Register event handler in the class Storage if eventhandler is already existed.

If at least one event handler is already registered,
the API returns the storage state from a variable within the class.
Otherwise, it retrieves the status externally.

In the previous implementation, an event handler was only registered within class Storage
when it was registered for each storage ID,
and no event handler was registered within class Storage when it was registered by storage type.
As a result of that, when the USB (or SD card) was removed,
it attempted to obtain the state from outside and provided incorrect information
to the application because there is no storage already removed.

Signed-off-by: Unsung Lee <unsung.lee@samsung.com>