Vendor & layer paths on Android

Currently the common default path for Vendor ICD or layer files is `/system/vendor/Khronos/OpenCL/vendors`

If Android's [Soong](https://source.android.com/docs/setup/build) build system is used to install ICD and/or layer files on Android (e.g. by using `prebuilt_root`), a custom directory under `/system/vendor` cannot be created. As such, it would be beneficial if the common path could be updated for Android to support users leveraging Soong for these files.

`/system` is also simply a symlink to `/vendor` so that prefix could be removed at the same time.

An initial suggestion allowing Soong's `prebuilt_etc` (where subdirectories can be created) rule to be leveraged would be to set the paths to `/vendor/etc/opencl/{drivers,layers}` either as the default Android path, or as a build option to maintain compatibility with any shipping configurations.

Start by locating the Android-specific default paths and build-option handling in OpenCL-ICD-Loader; the issue does not name files or tests. Compare the proposed `/vendor/etc/opencl/{drivers,layers}` paths with the existing `/system/vendor/Khronos/OpenCL/vendors` layout and Soong's `prebuilt_etc` constraints. Done means Android installations can use the supported path without breaking existing configurations.

Written by the indexing model from the issue text.