Use RAII to ensure KeyMint keyblobs deleted

If some check in a VTS test case fails, the test function may exit early
and not call CheckedDeleteKey(&some_keyblob), thus "leaking" a key blob.

This isn't normally an issue, but if the key blob happens to use a
feature that uses some secure storage (e.g. ROLLBACK_RESISTANCE or
USAGE_COUNT_LIMIT=1) then this may leak some scarse resource.

To avoid the chance of this, use an RAII holder to ensure that
manually-managed keyblobs (i.e. key blobs that are not held in the
key_blob_ member of the base test class) are always deleted.

Bug: 262212842
Test: VtsAidlKeyMintTargetTest
Change-Id: Ie8806095e249870484b9875eb660070607f339a3