m3: refine watch default determinate progress bar

Replace "ring" with "arc" shape, which allows rounded stroke line cap.

Before: https://b.corp.google.com/issues/380000245#comment1
After: https://hsv.googleplex.com/4812304171401216

Flag: EXEMPT. xml change
Test: manual
Bug: 380000245
Change-Id: I588213b02fad7b416b963dabd929b7ca53f12122