SurfaceFlinger: use std::set for DisplayConfigs

Use std::set instead of std::unordered::set in order to keep
the configs sorted. This is useful as we would return a sorted
vector from getAllowedDisplayConfigs() and the unittest expects that.

Test: adb shell /data/nativetest64/SurfaceFlinger_test/SurfaceFlinger_test --gtest_filter=DisplayActiveConfigTest.allConfigsAllowed
Fixes: 130168276
Change-Id: Ifc1f2be561193d93032e99eb10faa7dcce1ff21d