liblog: Add android_set_log_frontend

android_set_log_frontend allows one to select LOGGER_DEFAULT,
LOGGER_LOGD or LOGGER_NULL, the latter allows a runtime mechanism to
turn logging on or off.  LOGGER_DEFAULT is LOGGER_LOGD.

Test: gTest liblog-unit-tests notably liblog.android_set_log_frontend
      and liblog-benchmarks before and after.
Bug: 27405083
Change-Id: I365dbee9f7d83112da0ef8d1cace748d247773c8