Set stem and suffix for vndk prebuilt libraries

This is to pass check_elf_files when vendor modules link against vndk
prebuilts. Because of the ".vendor" suffix, the intermediate path
becomes "libfoo.vendor.so" and it makes check_elf_files fail, when stem
and suffix is not specified.

Also, by adding such fields, devices with empty BOARD_VNDK_VERSION can
experience a build break due to collision between VNDK snapshot and VNDK
source module. To completely prevent such collision, all vndk snapshots
become explicitly uninstallable, and they are disabled if
BOARD_VNDK_VERSION is empty.

Test: build/soong/build_test.bash
Change-Id: I54a0f33fd0b84ab9376ee3d75b83113b94bbacae