Correct proc file reader, optimizations.

Moved away from BufferedReader, which only reads the first 8KB of
some proc files because it aggresively fills its buffer.  Optimized
proc parsing, now double the speed.  Tests to cover.

Log when NetworkStats counters roll backwards when subtracting, and
optimizations around findIndex().  When system removes UID, also
remove from last stats snapshot to avoid xt counters from rolling
backwards.

Bug: 5472949, 5458380
Change-Id: I07c08fe5233156fac2b84450f6291868bf9bfaf2