Rootless GPU Debug

This commit adds a new toggle to allow loading GPU debug
layers from the base directory of debuggable applications.

It corresponds to changes to GraphicsEnvironment to look for
matching application and layer list, which are then selectively
passed on the native components, like the Vulkan loader.

Bug: 63708377
Test: Manual, CTS tests to follow
Change-Id: Id6917a7b34df4a37905fafe08a4c1b6503abbb0c