RenderThread: Make ~IFrameCallback() virtual

IFrameCallback has a pure virtual method, but its destructor was
non-virtual.  While CanvasContext, which inherits from
IFrameCallback, declares its own destructor virtual, this is not
sufficient when accessing via an IFrameCallback pointer.

We fix this by making the base destructor virtual.

Test: Treehugger
Change-Id: I6424f90c16caea417b5f80b60d76a7ca7c7ef97c