Intent to manage subscription plans.

Define an API for carrier apps to provide a deep-link into their app
for the user to see more details about their billing relationship,
such as upgrading plans.

The createManageSubscriptionIntent() method is ready for OS
components to use when deciding if they should show a "MANAGE"
button in their UI, and the returned Intent is fully constructed
and ready to roll.

Test: builds, boots
Bug: 64133169
Change-Id: I9b2775b7cba5313f517996870135eb4682082873