Prepare KeyChainSnapshot to removing deprecated getTrustedHardwarePublicKey method.

Add null checks to getTrustedHardwareCertPath.
Remove unused and outdated PersistentKeyChainSnapshot class.
Use CertPath instead of public keys in  KeySyncTaskTest.

Bug: 75952916
Test: adb shell am instrument \
-w -e package com.android.server.locksettings.recoverablekeystore \
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Change-Id: Ifabe7d5fa250069ebe0885ce52ec29b01294f63a