Persistence layer for quick affordances.

This approach uses SharedPreferences to persist the selected affordance
IDs for each slot.

Fix: 254858695
Test: unit tests but also manually verified that selecting some
affordances and seeing them on the lock screen then killing the system
UI process via adb and locking the screen again still showed the
selected affordances as expected.

Change-Id: If95af61c7beb14ce97e08018ce7c3b5eed6a06d6