Move extra VNDKs to /system_ext

Extra VNDKs are now installed under /system_ext in APEX format with
a phony target "vndk_apex_snapshot_package".

There are still files remained in /system/etc(*.libraries.<VER>.txt)
which are installed with "vndk_snapshot_packages".

These files are already packaged into VNDK APEXes, but linkerconfig and
libnativeloader still use them from /system/etc.(b/145184886)

Bug: 137802149
Test: lunch aosp_arm64
  && flash system.img on Q device
  && boot

Change-Id: I94c340d6f1c1af6ab1ae93c22b0a98fd4c10262e