Small fixes for Edit mode

Includes:
- Removing the "Remove" pill above the current tiles grid
- Showing available tiles when dragging within the current tiles grid
- Adjust the dragging zones on large tiles to avoid visual jank
- Adjust the color for the back arrow in the top left corner

Fixes: 404584621
Fixes: 404884846
Fixes: 404877414
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Test: manually
Test: DragAndDropTest
Test: EditModeTest

Change-Id: I53ec9c3bfd8ab81a03ce391e2026e25f8732c577