Keep smartspace session object

The object was getting prematurely optimized out because it wasn't referenced
in code later, but it is necessary to keep alive in order to get
recommendation updates.
Add annotation to make sure it's kept, and explicitly close when destroy
is called

Bug: 273739913
Bug: 283493228
Test: manual - verify session is not destroyed after restarting SysUI
Change-Id: I98bd7ce5878f9f261e1c09673faaae35e954f173