Make libnativewindow as LL-NDK

VNDK and vendor code can now link against to libnativewindow which
provides AHardwareBuffer that is an alternative to GraphicBuffer.
GraphicBuffer is provided by libui.so and the lib is not available to
SP-HALs and not encouraged for ordinary (binderized) HALs.

Bug: 37488074
Test: BOARD_VNDK_VERSION=current m libnativewindow builds successfully
Test: libnativewindow.so is only installed to system/lib(64)/ and not
/vendor/lib(64).

Change-Id: Ibc56e13d1a048f2270741fa6aec0687cfeac3598