ndkstubgen: use llndk=<version> for new llndk stub

We want LLNDK symbols to be explicitly marked with llndk tag to
handle LLNDK freezing which happens before SDK freezing. If symbols
need to be frozen as LLNDK, those symbols must be marked explicitly with
correct vFRC version.

In the following example,

LIBFOO { # introduced=35
  foo;
  bar;
  bar; # llndk=202404
  baz; # llndk=202404
  qux; # llndk=202505
};

NDK libfoo will have foo and bar while LLNDK libfoo stub will have bar
and baz for 202404.

Bug: 329012338
Test: test_ndkstubgen test_symbolfile
Change-Id: I384f589b240fa047e8871964bf9550f426024dfc