CompatibilityMatrix::combine add <kernel> from new matrices

Just as <hal> and <xmlfile> are added from new matrices
to old matrices as optional, so are <kernel> tags as well.
This avoids the hack that new <kernel> versions are added
retroactively. Specifically, we don't want to add 4.14 to
every matrix, but rather let libvintf do this automagically.

Bug: 78576469
Test: vintf_object_test
Test: libvintf_test
Test: manually inspect built matrices
Change-Id: Ia779c5e25c7c3db218007d23e4c887e8cd44e730
3 files changed