Rootless GPU Debug

Add the ability to load GPU debug layers from the base
directory of debuggable applications.

This commit:
* Adds a new Setting to Developer options: "Enable GPU debug layers"
* Adds a new way to discover and specify GPU debug layers per app
* Moves much of the layer enabling logic into GraphicsEnvironment
* Removes the JNI component of ApplicationLoaders

Bug: 63708377
Test: Manual, CTS tests to follow
Change-Id: I7d33e9f835f49aa8d86e63bdb32037728bc8b6a4