APIs to profile network usage for current UID.

Added startDataProfiling() and stopDataProfiling() to TrafficStats,
which can be used by apps to measure network usage delta between two
points in time.  Currently takes two NetworkStats snapshots and returns
delta, which will eventually include tag-level granularity.  Added
tests for NetworkStats delta subtraction.

Added NMS.getNetworkStatsUidDetail() that returns stats for specific
UID.  Always gives stats access for the calling UID, otherwise enforces
that caller has permission.  Fix readSingleLongFromFile(), since
/proc/ files don't have well-defined lengths.

Change-Id: Ic5b6414d8effbd66846e275b00d4b8a82c74589d