Fix a race-condtion in SurfaceFlinger that could lead to a crash.

Client::mLayers could be accessed from different threads.
On one side from Client::attachLayer() which is currently
called from a binder thread; on the other side from
Client::detachLayer() which is always called from the main
thread.

This could lead to a corruption of Client::mLayers.

We fix this issue by adding an internal lock to Client.

Change-Id: Ib1317d7750ed5030e6f577efe34b69fc10198bd3