Add config for overlaying extra default QS tiles

The new config resource allows for an overlay to specify additional QS
Tiles. These tiles will be available on a fresh flash of the device, as
well as when the user resets the tiles in QSCustomizer.

If the tiles are custom tiles and the apk is not installedi or the
TileService is disabled, the tile will just be discarded.

Test: atest QSTileHostTest
Test: add the grayscale custom spec to the config value and check that
the tile appears on reset.
Bug: 147493661
Change-Id: Ifbb8cc8ac7ef16c4b67ebbc776703e9ac8208c43