Add RecoverySession importKeyChainSnapshot method

This imports the keys directly into the keystore of LockSettingsService,
allowing them to be accessed via the RecoveryController getKey method.
This is better as it does not expose raw key material to any app.

Bug: 74345822
Test: runtest frameworks-services -p \
      com.android.server.locksettings.recoverablekeystore

Change-Id: I4991b0cff1d2fa2e5bd0b53a71c096499e93e98b