Merge "KeyMint: Document KeyCharacteristics" am: 30a6b0e7f0 am: cd489e35d3 am: 241295940e

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/1736773

Change-Id: Ia3b606456a93840cc0eb547d2c8b8f29b88aa9b9