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.

Bug: 4483046

Change-Id: I5262bf1124d9a65ec6f8ffd8e367356fc33a7536