liblog: Add C++ wrapper for C event log handler

Add android_log_event_context class.

Test: in context of 'logd: add getEventTag command and service'
Bug: 31456426
Bug: 26552300
Bug: 31289077
Bug: 19235719
Change-Id: Icf918b443f01e04040c8d3efe0cdb7a8e70be10c