Refactor PlatformKeyManger to remove mUserId to fix two tests that fail

on clean devices without secure screenlocks

The mUserId is removed to allow a mocked PlatformKeyManager to be passed
into RecoverableKeyStoreManager for tests.

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

Change-Id: I9b1fda3c7d869b683cd71cb81ea88da5d995ace9