Add policy transparency for metered data related settings.

With API Dpm.setMeteredDataDisabled, admins can restrict some
packages from accessing metered data and when admin does restrict
packages, user settings to enable usage of mobile data in background or
allow unrestricted access during data saver mode are not relevant. Add
policy transparency to these settings so that user knows that admin
disabled them.

Bug: 63700027
Test: make RunSettingsRoboTests
Test: manual
Change-Id: I450f7a91356ed8fb33f464620c73fa9407a1ff83