Use RecoverySession object to hide session IDs (redux)

Session IDs are an implementation detail that the framework can (and should)
abstract away. This was previously reverted due to breaking master.

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

Change-Id: I4427c818348c054ada39d799b6da3b739f27eba9