Update the drag to dismiss UI to be closer to design

Rather than a circular target for drag to dismiss, there is text / icon
as well as a gradient. As the PIP approaches the text / icon they grow
in size. If the PIP is released overtop of the text / icon, it is
dismissed.

Test: Using PIP test app, have a PIP, drag it to dismiss target area.
Change-Id: I339ad14e144dfd61f0e990ba4d2559642a47b141