Implement RecoverableKeyStoreLoader loading keys into users' keystores

The loader needs to be able to save keys into a particular app's keystore. This
ought to work as the service will run with root privileges.

Test: adb shell am instrument -w -e package android.security.recoverablekeystore com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I9722df361ccc93cbf2afb73ea09f8c7af687eaff