Augment network stats based on SubscriptionPlan.

When a carrier provides an "anchor" of data usage at a specific
moment in time, augment the network statistics used by warning/limit
thresholds and Settings UI.  For example, if the OS measured 500MB
of usage, but the carrier says only 400MB has been used, we "squish"
down the OS measured usage to match that anchor.

Callers using the hidden API will have their data augmented by
default, and the public API offers a way to opt-into augmentation.

Thorough testing to verify behavior.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.
Test: cts-tradefed run commandAndExit cts-dev -m CtsUsageStatsTestCases -t android.app.usage.cts.NetworkUsageStatsTest
Bug: 64534190
Change-Id: Id3d4d7625bbf04f57643e51dbf376e3fa0ea8eca