Create aconfig flag for relocating Smartspace to bottom.

Per recommended workaround in go/sysui-aconfig, "all flags in each
codebase should use just one namespace". Therefore, this CL uses
systemui.aconfig rather than sysui_integrations.aconfig.

Bug: 316212788
Flag: N/A
Test: N/A

Change-Id: I403a3ac37ca9c0002c22d923390ba4f77e334e50