Add getKeyCharacteristics stub to KeyMint

Add a stub implementation of getKeyCharacteristics which simply returns
UNIMPLEMENTED. A future change will replace this with a real
implementation.

(cherry picked from commit 8fa89b5538eb822790f0fb100e506d3b003d393d)

Bug: 186685601
Test: Treehugger
Ignore-AOSP-First: Implementation exists in internal master only
Merged-In: I5b33a48b90c0b7e28eb96ebbe099099661fc56de
Change-Id: I5b33a48b90c0b7e28eb96ebbe099099661fc56de
2 files changed