surfaceflinger: use std::queue for mPendingEvents

Replace Vector by std::queue for mPendingEvents.  It is used for
pending hotplug events, which happen rarely and are consumed one by
one quickly by an RT thread.  std::queue is likely no better than
std::vector in that scenario, but is more expressive.

Bug: 115738279
Test: boots
Change-Id: Ia906a7f08ecbe6d5534cd6ec42e5af2ac865a911