runtime: Handle prebundles on BOARD_NEEDS_VENDORIMAGE_SYMLINK.

  If a target device (ex: angler) sets BOARD_NEEDS_VENDORIMAGE_SYMLINK,
  the prebundled applications within the system.img will be extracted
  under `system/bundled-app`. Handle this scenario by setting the right
  PREBUNDLED_ROOT environment variable on boot, based on existance
  of a file.

Change-Id: I25a7d6638b0032ecff4bc37124bb2e378950c3b2
TICKET: CYNGNOS-2092