Modify comments for SurfaceSession.

SurfaceSession is allowed to be null, and if it is null, it will get
the instance of SurfaceComposerClient via
`SurfaceComposerClient::getDefault()` in JNI.

Change-Id: Icbeb47d1be3d443a98bae4ef3b7feba49ebdbbe8
Signed-off-by: jiayongqiang <jiayongqiang@xiaomi.com>