Fix circle in the opa ring and enlarge circle when opa is disabled (1/2)

Use vector drawables to show the ring and the circle instead of using
shapes because for some devices, the scaling causes the circle to not
sit in the center. When opa is disabled, the home button will enlarge to
the full size.

Test: manual - enable assistant and look at home button on pixel devices
Change-Id: If582d5f4dd5b3c8f67156bc6b1b1e40e183e0ed2
Fixes:112603227