Sync public.libraries.buildbot.txt to ART chroot directory.

Have `art/tools/buildbot-sync.sh` overwrite the default
public.libraries.txt file with a smaller
one (`art/tools/public.libraries.buildbot.txt`) that contains only the
public libraries pushed to the chroot environment on device.

This is normally not needed for ART Buildbot targets (the ones defined
in `device/generic/art`), as they use `PRODUCT_COPY_FILES` to install
`art/tools/public.libraries.buildbot.txt` as
`/system/etc/public.libraries.txt` on device. However this is needed
for other targets, as they normally install
`system/core/rootdir/etc/public.libraries.android.txt` as
`/system/etc/public.libraries.txt` on device.

Test: ART chroot-based on-device testing using the master-art branch
Test: ART chroot-based on-device testing using the master branch.
Bug: 124425036
Bug: 121117762
Change-Id: Ib7a570344e26d448759a355a99a4a50089a0a3ce