GPU Debug Layers: Allow multiple layer apps

This allows applications to specify both a GLES layer app and a
Vulkan layer app, so both APIs can be debugged at the same time.

To specify multiple debug layer apps, colon separate them just
like the layers:

  adb shell settings put global gpu_debug_layer_app app1:app2:appN

Bug: 110883880
Test: atest CtsGpuToolsHostTestCases
Change-Id: I5f721b7d9de59577adb6cc03e909ff99ca79d75e