Prevent no tiles in QS

In some cases, normal operation can lead to no tiles in QS (for example,
when restoring from a non-AOSP device that uses different specs for
tiles). Unfortunately, as the tiles use unknown names, we are not able
to restore the correct tiles.

If after processing the tiles in CurrentTilesInteractor, we end
up with no tiles, we revert to the default.

Note that this should only happen when things directly try to set the
list of tiles. A user manually adding/removing tiles will never
encounter this scenario,

Test: atest NoEmptyTilesTest
Test: manual, set tiles to unknown names
Fixes: 324575996
Flag: ACONFIG com.android.systemui.qs_new_pipeline NEXTFOOD

Change-Id: Ie16a2fc9a57680c1860953e6b82b265e269047a1