Add recovery flow utils to KeySyncUtils

Currently KeySyncUtils only contains methods for the backup to cloud
flow. This adds utility methods for the restore flow, also. We still
haven't merged SecureBox, so the class they defer to at the moment
throws for most of these methods, meaning we can't yet add meaningful
unit tests.

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I27fe8b33dd239488b11f2863b67af4575fa82b47