Interpolate radius when animating flyout to dot

Some themes have very small corner radius defined (e.g. square icons) and
when animating it does not look very good. This CL interpolates the value
with the dot radius to have smoother animation.

Test: manual - use square theme and post a bubble, observe flyout
Bug: 129158983
Change-Id: I355965bcdbda1852784ea597d4a60263e4b08286