Update recoverKeys to return raw material

This is a temporary solution, while the KeyStore team works on adding a
move API to KeyStore. (At which point this will be updated to instead
return 'move tokens', allowing the user to move the key from the system's
keystore to their own, without ever seeing the raw material.)

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I2241a6da15d50c26a7b384d4e5b6f78366fb9300