Use Vector instead of unordered_map for hotplug displays

For multi-display order of the hotplug matters, and vector
maintains the order that we need for the multi-display.
see: ag/1921760 for partner cl on HIDL.

BUG: 210920960
BUG: 209409863

Test: atest VtsHalGraphicsComposer3_TargetTest
Change-Id: I4f9a86413f20c860fc0bc3850a14335d62de881a