liblog: add log/log_id.h

Move common log_id_t and simple internal support function definitions
that use it from log/log.h to log/log_id.h.

log_id_t
__android_log_buf_write
__android_log_buf_print
android_name_to_log_id
android_log_id_to_name

Test: gTest liblog-unit-tests --gtest_filter=\
      liblog.log_id:\
      liblog.__android_log_buf_print:\
      liblog.__android_log_buf_write:\
      liblog.__android_log_buf_print__concurrent64
Bug: 34250038
Change-Id: Iad3704cc72943a3094e1193a6d032c7f29a6cd5c