Add key encryption to KeySyncTask

This mostly completes the task. Still some TODOs around constructing
the vault metadata correctly, and exactly what the Intent should look
like.

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: Ibf5705830f577597f0cc2afdd906f6517e426d01