[flexiglass] Fixes exception when switching users.

When switching users, makes sure that the CoroutineScope associated with the old DataStore instance is fully cancelled before creating a new one.

Fix: 411761375
Test: Switched to secondary user and back while filtering logcat for
"DataStores". Before the change, I saw the message when switching back
to primary, after the change, it's not there.
Test: Made sure that I still get educational tooltips on the dual shades
and I get them separately between the two users.
Flag: com.android.systemui.scene_container

Change-Id: I84f01661dd0fd8b9872f1c5f0ad18b9493934aa4