KeyMint VTS: use GTEST_SKIP for optional fn

When a KeyMint VTS exercises optional functionality, where possible
use GTEST_SKIP() when that functionality is absent, so the test
summary includes information about what is present and what isn't.

This should not affect the overall test result.

Test: VtsAidlKeyMintTargetTest
Change-Id: I62d244d2e4ecc67737906009575e64b50450d4c4