Make libgui available to vendors

libgui is now available to vendors, especially for BufferQueue in
between the OMX and Camera HALs for encoding the captured video stream.

In doing so, its headers were moved from frameworks/native/include to
its local directory frameworks/native/libs/gui/include. This is required
because global include path is not supported when building for vendor.
However, in order not to affect platform modules that are still relying
on the global include path, symlinks are provided.

frameworks/native/include/gui -> frameworks/native/libs/include/gui
frameworks/native/include/private/gui ->
frameworks/native/libs/include/private/gui

Bug: 37731063
Test: BOARD_VNDK_VERSION=current m -j libgui.vendor
Change-Id: I0bab8d2e31959085ad58b1ce4c610334ee3da033