Fix tv pip menu rounded corner artifacts

While the tv pip menu edu text is showing, we draw a "pip border"
around the pip content to draw rounded corners on top of the
non-rounded corners of the pip surface.

Separately, we draw a "pip frame" around the whole pip menu, including
the edu text, which turns white when the pip menu is focused.

The pip frame and the pip border have the same width and rounded corners
values. When the edu text closes, the pip border and the pip frame
overlap exactly and theoretically, the pip border should not be visible
behind the pip frame. However, when the pip frame is highlighted, the
pip border is slightly visible at the rounded corners.

To mitigate this, in this CL we change the visibility of the pip border
to GONE when the edu text animation ends. At that point the pip border
and the pip frame overlap exactly, so there is no need for the pip
border anymore.

Bug: 275693800
Test: manual

Change-Id: If3cceb8e01253ae2fb1ffd37dd2287927ad4ac58