Add PersistentKeyChainSnapshot serialization/deserialization methods.

Unlike Parcelables, Byte array produced by the class can be safely stored in the database.

Bug: 71804644
Test: adb shell am instrument -w -e package
com.android.server.locksettings.recoverablekeystore
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Change-Id: I826a0cc4d7dc33ff1a062374a4fc8471db8e2f34