APIs to disable data usage notifications.

Certain carriers have requested the ability to disable the warning,
limit, and rapid data usage notifications shown by the OS, so give
them that ability on a per-subId basis.  (The notifications are still
enabled by default.)

Unhide NOT_CONGESTED to match setSubscriptionOverrideCongested().

Fix bug in getPrimarySubscriptionPlanLocked() to find the first
currently active plan; the first non-recurring plan may be no longer
active.  Fix bug in SubscriptionPlan which allowed invalid negative
or zero-length Period objects.

Bug: 77851691, 77506882, 77662747
Test: atest com.android.server.NetworkPolicyManagerServiceTest
Test: atest android.telephony.cts.SubscriptionManagerTest
Exempt-From-Owner-Approval: SDK deadline, responding to API feedback
Change-Id: Ib6e6dbadd5022937c3f0661ab4b66aa617f0b5af