Backup & Restore support for quick affordances.

Adds support for backup & restore of the currently-selected quick
affordances on the lock screen.

Only supports user 0 for now. A future CL will add support for additional users since the feature is broken for multi-user anyway (see b/260251307).

Fix: 258278136
Test: added test case. Manually verified, using the instructions from
go/localtransport that selecting some affordances, then triggering a
backup, then changing the selections and triggering a restore properly
restores the initial set of selections.

Change-Id: I9f2a51287a7467af95e7824f2b1804baba962099