Remove path prefix within VNDK snapshot zip

Remove 'android-vndk-snapshot' path prefix when creating VNDK
snapshot zip with soong_zip. The path prefix makes it cumbersome
to unzip and install the snapshots to prebuilts/vndk/v{version}.

Test: development/vndk/snapshot/build.sh &&
python development/vndk/snapshot/update.py
Bug: 69950927
Change-Id: I9e5a1898056627ba98a408f048b7cb710e5ef07c