Only load native EGL library if using GL back-end

In line with recently adjusted goals, remove application support
for specifying the ANGLE backend. That allows us to move the
backend property check into the Loader and only load the native
EGL library if the backend property indicates so.
Bug: 80239516
Test: manual, verify log only shows native library loaded when
ANGLE GL backend is selected.
    Select application to use ANGLE via Developer options
    adb shell setprop debug.angle.backend 0
    Start app.
    Review logs for "libEGL Loaded: libEGL_<native_library>.so

Change-Id: Ifae9b62ecf6723583bd461305f5a7e95910cfa43