Update SubInfo#getNumber docs to reflect permission requirement

getLine1Number has been moved out of the READ_PHONE_STATE permission,
but callers with READ_PHONE_STATE can still query SubscriptionInfo
which contains the number for the subscription. The number will be
sanitized from the resulting SubscriptionInfo object(s); this change
updates the docs to reflect the new READ_PHONE_NUMBERS permission
requirement.

Bug: 152063877
Test: atest SubscriptionControllerTest
Change-Id: If4834ad0f648556207ad9b848fc4db332993a6d6