libvulkan: Update global layer loading rules

* Look for layers in /data/local/debug/vulkan instead of
  /data/local/tmp/vulkan.
* Only enumerate global layers in debuggable processes.
* Only allow force-enabling layers and callbacks in debuggable
  processes.

Change-Id: I1f41a03ecbb0cc159e7dbb0b424005559996c196
(cherry picked from commit e784c114bbdc32361096cce8f59eb9513390e2d6)