Add storage for platform key IDs to SQLite db

Also fix UNIQUE constraint for keys table and add test.

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I868cc4385b6557135ef1d40b39f23c0383453ca3