Mark libui and libgui as VNDK in Android.bp

As a VNDK module, Android.bp must have 'vndk' tag as well as
'vendor_available: true'.

The 'vndk' tag for VNDK module is formated as below:
vndk: {
    enabled: true,
},

VNDK modules will be installed both in system/lib(64) as normal and
in system/lib(64)/vndk as a vendor variant.

Bug: 63866913
Test: build and boot with BOARD_VNDK_VERSION=current
Merged-In: Idf46c030e42a70c3b97a6d6a12a1087707fe7615
Change-Id: Idf46c030e42a70c3b97a6d6a12a1087707fe7615
(cherry picked from commit 71d6c885)