Fix vndk snapshot suffix on Makefile

When exporting Soong modules to Makefile, each VNDK snapshot module
whose version is BOARD_VNDK_VERSION has ".vendor" suffix. So makeLibName
should also return ".vendor", not the full vndk snapshot suffix like
".vndk.29.arm64.vendor".

Exempt-From-Owner-Approval: cherry-pick

Bug: 151274994
Test: manual build with snapshot
Change-Id: I48f606f90f6c5ca55d0a5bc29bad491c38a44f47
Merged-In: I48f606f90f6c5ca55d0a5bc29bad491c38a44f47
(cherry picked from commit 7ac1fa78)