Ensure TileQueryHelper#mTiles is thread safe.

- Ensure TileQueryHelper#mTiles is only modified on the background
thread and a send a copy to listeners.
- Do not call onTilesChanged until all stock tiles are loaded (as
opposed to after each tile) to reduce tile spec recalculation.
- Remove completion callback and instead maintain query state in
TileQueryHelper

Bug: 75415415
Test: manually
Change-Id: I2a20bd916dae0ee9f4c422ec4e2ac3f2d47da3dc