Polished rapid data usage alerting.

Switch to reading limit information from NetworkPolicy, which is
typically populated from SubscriptionPlan.  This lets users have
direct control over the limits we're using to trigger rapid usage
alerts, and makes the feature work without requiring that the carrier
wire up SubscriptionPlan information.

Let the user "snooze" the rapid usage alerting for a day at a time,
so we're less annoying to them.  Send the snooze broadcasts as
foreground, so that we don't re-post notifications while working
through a long background broadcast queue.

Fix notifications to use the "ALERTS" channel, since these alerts
really are higher priority than simple "STATUS" updates; this also
gives us HUN behavior when in full-screen apps.

Update both service and unit tests to work directly with
NotificationManager, instead of the raw AIDL.

Test: bit FrameworksServicesTests:com.android.server.NetworkPolicyManagerServiceTest
Bug: 72444638, 72436702
Change-Id: I8d9138522a7779cc68eb9fa4777b50facb6567b7