[sugar] Activity API changes

[Marco Pesenti Gritti]

Hello,

I refactored the activity API quite a bit today. Since no one is using
the presence service yet, the required changes will be trivial.

1 Change the import since the module name is changed. Something like:

from sugar.activity import activity

2 Change the constructor to take and pass to Activity the handle
parameter. Something like:

class MyActivity(activity.Activity):
    def __init__(self, handle):
        activity.Activity.__init__(handle)

I *might* also do a trivial change to activity.info tomorrow, while we
are breaking stuff.

If there is any issue please let me know.

Marco