more clean-up of Surfaceflinger's client management

SurfaceComposerClient now only exist on the WindowManager side,
the client side uses the new SurfaceClient class, which only
exposes what a client needs.

also instead of keeping mappings from IBinder to SurfaceComposerClients
we have a SurfaceClient per Surface (referring to the same IBinder), this
is made possible by the fact that SurfaceClient is very light.

Change-Id: I6a1f7015424f07871632a25ed6a502c55abfcfa6