Zygote: do not preload Vulkan driver in Q

This is a partial revert of http://ag/7064585 to be safe for Q.

1. skia is not going to use Vulkan backend in Q.
2. usap is no longer preloading graphics drivers.
3. Most importantly, Vulkan loader is not quite ready to be preloaded in Zygote.
   Because the layers discovering mechanism currently only happens once with the
   driver loading. By preloading in Zygote, apps won't be able to use the layers
   shipped with the debug apk.

Bug: 135281983
Test: build, flash and boot.
Change-Id: I88b0ad60ab8233714253a92166da76dc992f7f98