[1/n] Add WALLPAPER_PICKER_GRID_APPLY_BUTTON to SysUi Flags.kt

Resubmit of ag/24343660 that was reverted due to failure to merge on main.

Test: N/A
Bug: 294866904
Change-Id: Ibd45b0039959d24308a47bde4652ec768e7d9ceb
2 files changed