Add GLES specific setting for layers

This allows loading layers for GLES and Vulkan at the
same time by adding a GLES specific setting:

  GPU_DEBUG_LAYERS_GLES

which mirrors the existing setting:

  GPU_DEBUG_LAYERS

The Vulkan and GLES loaders now scan distinct layer
lists, correcting an issue that would prevent Vulkan
from loading when it failed to find GLES layers.

Bug: 110883880
Test: Load a GLES layer when running ANGLE with Vulkan backend
Test: cts-tradefed run singleCommand cts -m CtsGpuToolsHostTestCases
Change-Id: I9902fe7921a171cd779fd7dd957dd51e00b4ef02