Add KeySyncUtils

Static methods to help with the RecoverableKeyStoreLoader remote sync
flow.

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: Ibd5a8f6c9ee2d4d118a9e6be9b813e192205d6dc