fix VtsHalGraphicsComposerV2_2TargetTest timeout failed

run VtsHalGraphicsComposerV2_2TargetTest will timeout failed
when there is dual displays. use `std::unordered_set` Display type,
`std::vector<Display>(mDisplays.begin(), mDisplays.end())` as
getDisplays return value, mDisplays elements order may not be
same as the order of hotplug events during boot matters.
We should store `mDisplays` as `std::vector`, since the order of
hotplug events during boot matters in determining the primary display.

Bug: 209409863
Test: run vts -m VtsHalGraphicsComposerV2_2TargetTest

Signed-off-by: Kui1 Wu <wukui1@xiaomi.com>
Change-Id: I628ad593640f28fb8a916822a91ac2c578cc299d