Use shared surface session for snapshot surface

So when drawing snapshot starting window, it doesn't need to
have an extra IPC ISurfaceComposer::createConnection to
surfaceflinger every time.

WallpaperService/SurfaceControlViewHost also use that, so the
instance SurfaceComposerClient::getDefault() is usually already
available to use directly.

Bug: 297502610
Flag: EXEMPT reduce unnecessary invocation
Test: Task snapshot starting window is normal from hot launch app.
Change-Id: I644aaca1d0cc88bb2a6969381b5a373dc9280ae1