Modes icon picker: measuring improvements

Ensure that horizontal spacing between items in the icon picker is distributed to the left and right of the row instead of between the items. This makes the horizontal and vertical spacing the same.

Bug: 359171199
Test: manual + hsv
Flag: android.app.modes_ui
Change-Id: I68c3860ba4092bb6fde2a1218b3f6a95d0c373da