Load vulkan driver from an apex when ro.vulkan.apex is set

When ro.vulkan.apex is set, loader tries to load driver from the apex. This
enables vulkan driver to be packaged in APEX.

Bug: 205618237
Bug: 306294280
Test: cvd start -gpu_mode=guest_swiftshader
      # load vulkan.pastel from apex.
Change-Id: I63feb6d55fb9918b95d3cff0d9ffa0d004e94645