Use libvendorsupport for vendor api level calculation

Remove temporary 'vendor_api_level_of' function from init and replace
the function with the same in libvendorsupport.

Bug: 312403948
Test: getprop ro.vendor.api_level
Ignore-AOSP-First: Avoid merge conflict with ag/23874202
Change-Id: I095353e602397220571e131431e7cbd1b8511fa6