camera.provider@2.4: Legacy wrapper: Run with more threads.

Each concurrent camera device in use needs 1 thread to maintain
operational semantics, plus we probably need threads for sensor service
calls, misc camera service calls, etc.

Up thread count to 6. That should be enough for everybody.

Test: Camera still works on sailfish
Change-Id: I2262c69985756fbf762034663ce24f45bf7ce2f1