Make IKeyChainAliasCallback oneway

So it can be sent from devicepolicymanager (system_server) to keychain
(a system_app) without waiting on the response and having to do
everything in a background thread.

Side-effect: the regular keychain => app callback is slightly more
efficient now too. in case anyone particularly needs blazing fast
private key user selections.

Fix: 35675253
Test: cts-tradefed run cts --abi=arm64-v8a --skip-device-info --module CtsDevicePolicyManagerTestCases --test 'com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement' </dev/null 2>&1
Change-Id: I6e9d96ca3c42e6489d879d8cfb0507eb94838bf1