install VNDK APEX instead of VNDK libs

VNDK APEX replaces VNDK libs under /system/libs/vndk[-sp].

For current VNDK (vndk_package), com.android.vndk.current APEX is
installed instead of VNDK libraries.

For older versions of VNDK (vndk_snapshot_package),
com.android.vndk.v## APEXes are installed along with prebuilt VNDK libs.
The reason why phony targets of VNDK prebuilts are still installed is
that phony targets install the required *.libraries.##.txt files to
/system/etc.

After those .txt files are moved to APEXes, then we can remove those phony
targets.(b/141450808)

Bug: 141451661
Test: m && boot (tested with cuttlefish)

Change-Id: Ibfa06d42ec0081fa7010091ef097bb940bacf8d6