Reduce padding in Modes dialog and make marquee less eager

Also, controlled by a flag, switch to one-row-per-tile. This flag is not intended to be advanced, but only used to compare both approaches, and eventually inline the one we prefer.

Bug: 366034002
Test: manual
Flag: android.app.modes_ui
Change-Id: I94a4467ebd572d17250d78b9ef772152f58caa77