surfaceflinger: recompute visibility after sideband stream change

We normally recompute layer visibility when a layer gets its first
buffer; before then it's treated as invisible. Sideband layers never
get a buffer (as far as SurfaceFlinger knows), so never became
visible. Now we also recompute visibility when a layer gets a new
sideband stream.

Bug: 17752511
Change-Id: I84e150f196eb2eb7bcd2616248e5e3fa73624809