SF: Flatten display containers

Store displays and tokens in contiguous storage for cache-efficient
lookup and iteration, which also ensures that internal displays are
composited before external and virtual displays.

Bug: 182939859
Bug: 185536303
Test: simpleperf
Test: libsurfaceflinger_unittest
Change-Id: I1ae65c3e80567b65736bd189c263be5be34862e3