libsnapshot: Don't package snapuserd.rc in /vendor/etc.

This is happening accidentally because of a bug in Soong. In the
meantime, add a separate snapuserd.vendor_ramdisk that does not have an
init_rc section.

Bug: N/A
Test: manual test
Change-Id: I9dcafb681f209932fa13c5c55478f0e486e53306