Polish PiP round corner

Included in this CL
- Increase the corner radius from 8dp to 16dp
- Scale the round corner radius in transition

Tested with the following scenarios
- Enter PiP in button navigation mode
- Double-tap to resize PiP
- Pinch to resize PiP

TODO
- Round corner when enter PiP with auto-enter being enabled

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/gA2dQNBNTA8xFExWqmOkRY
Bug: 187441996
Test: manual, see video
Change-Id: I43db31f966007795351a8e2ca7dd08417f6f0df7