Safeguard against no tiles in QS

In PagedTileLayout:
* Make sure that each page displays at least 1 tile (never 0).
* Make sure that there's at least one page (even if it's empty)

In QSTileHost:
* If the new value of sysui_qs_tiles produces no tiles (but it's not set
to empty), set the tile set to the empty default
@string/quick_settings_tiles

Test: adb shell settings put secure sysui_qs_tiles not-a-valid-tile-spec
Test: atest QSTileHostTest
Fixes: 135023694
Fixes: 135677464
Change-Id: I1e5cf4d2688370001ecae87fc0272acecd48af73