Mark EGL/GLES libs as vendor_available

EGL/GLES libs are marked as vendor_available, otherwise vendors are not
able to use OpenGL. The libs were not simply declared as LL-NDK because
doing so causes the dual loading of libui.so in vendor processes;
platform version of libui.so is loaded since it is depended by libEGL.so
whereas there also is vendor version of libui.so in /system/lib/vndk.

When libEGL.so is built for vendors, 1) libgraphicsenv is removed from
its dependency list as the lib must not be available to vendors and 2)
eglGetNativeClientBufferANDROID is unusable since the function relies on
AHardwareBuffer_to_ANativeWindowBuffer which isn't available to vendor.

Bug: 37731063
Test: BOARD_VNDK_VERSION=current m -j libEGL.vendor
Test: BOARD_VNDK_VERSION=current m -j libGLESv2.vendor

Change-Id: I83a128b14a5d852bb0f2cd821a8e3f82c66a313f