Mark the modules 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: Icecb22ed2ed0f58c3168605d4cf64815e2dda750
Change-Id: Icecb22ed2ed0f58c3168605d4cf64815e2dda750
(cherry picked from commit 9b0ed729)