KeyMint HAL: restore getKeyCharacteristics() docs

Commit 40eb8f53 ("KeyMint AIDL tweaks") removed references to
getKeyCharacteristics() in docs, as it was a KeyMaster entrypoint
that wasn't present on the KeyMint HAL.

Commit dadb18dd ("Add getKeyCharacteristics method to KeyMint")
added the getKeyCharacteristics() entrypoint to KeyMint, as it
turned out it was needed after all.

This commit restores references to getKeyCharacteristics() in the
Tag.aidl documentation.

Test: VtsAidlKeyMintTargetTest
Change-Id: I860479554b85f4adfeddd4eee70a09cf5265c938