Update getSubscriptionPlans documentation

getSubscriptionPlans can return null if a plan has not been set for the
given subId yet. Change the annotation from @NonNull -> @Nullable and
update the documentation. SubscriptionManager#getSubscriptionPlans will
return the @NonNull plans instead.

Test: build
Bug: 213896944
Change-Id: I7b232a1a695d1db7cb0d0a177bb1090e9033b805