KeyChain: Adding methods for user-visibility.

Add to the KeyChain aidl two methods for getting and setting whether
a key can be selectable by the user or not.

See
https://googleplex-android-review.git.corp.google.com/#/c/platform/packages/apps/KeyChain/+/3199414/

Test: To be determined.
Bug: 65624467
Change-Id: Ib31a11ca432a5d29fdb8ed5349598dbff4bcb516