Make sure surface control is really valid before giving it to client

Checking mSurfaceControl!=null is not enough to ensure the surface is
valid. tryStartExitingAnimation() could destroy/save the surface before
app is stopped, if it decided the surface is no longer needed.

Use WSA.hasSurface() to check if the surface control is really valid.

bug: 30766796
Change-Id: I7724f1929d4ebc3f9a7a3b19eec638d7a7d7b910