Alert user on rapid/heavy data usage.

Now that we have accurate information about a user's carrier data
plan, we can alert them if the current usage patterns would end up
with a nasty surprise towards the end of the current billing cycle.

For example, a single abusive app could use 90% of the user's budget
within the first few days of a billing cycle, leaving the user to
limp along for the remainder of the month.

The simple algorithm here extrapolates to see if the average usage
over the last 4 days would be more than 150% of the data limit for
the full billing cycle.  This period is short enough to catch rapid
recent usage, but long enough to smooth over short-term habit
changes, such as a weekend getaway.  This was chosen after
backtesting the proposed algorithm against real-world data usage
from a handful of internal users.

Fix NPMS unit tests, and write new ones, but leave the existing
@Ignored annotation intact for now.

Test: bit FrameworksServicesTests:com.android.server.NetworkPolicyManagerServiceTest
Bug: 64133169
Change-Id: I0d394b133257e8569a9aa2631b57638839d870ce