Only run legacy split flicker tests when it is using legacy split

Ignore legacy split flicker tests if the system is not using legacy
split by checking config_useLegacySplit which is set to false since
sc-v2 or on jumbojack device. The flag should be deprecated along with
legacy split screen.

Bug: 189245318
Test: pass existing tests
Change-Id: Ic3dfbe849e7fcc9f25e44b7ff37ad00fdee60e9a