wifi: Fix unit test for LegacyLinkLayerStats

Due to the introduction of multi links, StaLinkLayerStats carries stats
for multiple links as a vector. For LegacyLinkLayerStats, only single
link will be available. So convertLegacyLinkLayerStatsToAidl() push the
stats for single link as the first element of the link vector.Fix the
unit test to index the first link.

Bug: 246988155
Test: hardware/interfaces/wifi/aidl/default/tests/runtests.sh
Change-Id: I8c9a8beca6bda6cf894edb616cdfe54f09d029f7