Add PlatformEncryptionKey (again)

Version 2 of this. The other change had to be reverted due to breaking
the build. This is almost identical, just with some additional fixes for
the database api.

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I02928a9351739673bdffec55013c6ee7789edc1c