Allow applications to query for foreground/background data usage.

Currently the NetworkStatsManager APIs allow applications to
query for their own data usage by UID and tag, but do not allow
applications to query by foreground/background state.

This is causing popular apps to resort to parsing xt_qtaguid
stats files directly. Because this is no longer allowed for apps
targeting P and above, provide replacement functionality.

This API allows apps to query for data usage for a given state,
but not to receive data usage broken down by state. This is
consistent with how the current UID and tag APIs work. It is also
not an undue burden on apps: there are currently only two states
of interest (FOREGROUND and everything else), and even if we add
states in the future, unmodified apps can still obtain total
traffic using STATE_ALL.

Bug: 72977484
Test: New CTS test added in other change in this topic.
Change-Id: Ic8c9194569ffd599b49e4a8197c5c2ea0ec3f7f7