Only run PipLegacySplitScreenTest when it is using legacy split

Ignore PipLegacySplitScreenTest 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: atest PipLegacySplitScreenTest
Change-Id: I6243f0b0990265c96867cb7ee87b6bd0b90330c7