logd: class hierarcy for Uid and Pid statistics.

Add EntryBase and EntryBaseDropped base classes for statistical
entries to inherit from. Abstract add(), subtract() and drop()
methods to common LogBufferElement in each for entry policy decisions.
Some move of details out of LogStatistics.cpp and place them into
LogStatistics.h. Add statistical add(), subtract() and
drop() methods to hash table to call entries for policy.

Bug: 19608965
Change-Id: Ib8a33a8fe28871ef165d1632c6546a5c606231e8