Update javadocs for API review

Move plan requirements from Builder to SubscriptionPlan
Null and empty network types do not have the same behavior.
Clarify where plan requirements is enforced

Bug: 143550313
Test: none

Change-Id: I80c3b4425f317d5b4e9c685878c866742b706b85
Merged-In: I80c3b4425f317d5b4e9c685878c866742b706b85
2 files changed