Implement generateAndStoreKey

For now just returns raw key material. In the future we will need to
change this to use the KeyStore move api. (Once that has been
implemented.)

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I8aee4da81f0f853503f570dae8d74e1d29f124cc