Tweak SettingsSpinner UI

1. Arrow icon size from 24 to 18
2. Round corner radius from 20 to 28
3. Height from 32 to 36
4. Set accentColor background to default view.
5. Set translucent accentColor background to default dropdown view.
6. Set dropdown view vertical offset to measured height.

Bug: 181833196
Test: manual visual
Change-Id: Ia538fd1831602ff38ba873c7f0a34d1ad5e90e18