Align KeyMint AIDL with usage

 - Make HardwareAuthToken nullable on begin()
 - Drop unused vestigial performOperation() entrypoint
 - Drop unused Tag::BLOB_USAGE_REQUIREMENTS

Test: TreeHugger, VtsKeyMintAidlTargetTest (CF)
Change-Id: Ief6a9c97456cdf7e7626ff26a875792584c40199
6 files changed