liblog: add private android_log_write_list_buffer()

Allows us to compose an event message for our own in-memory purposes.
Will be used to compose an event message in logd and directly write
it to just the pmsg buffer.  Provide an internal enhanced C++ wrapper
for event handling.

Test: gTest liblog-unit-tests --gtest_filter=liblog.android_log_write_list_buffer
Bug: 31456426
Change-Id: I98246898ba580f9e506baba8af2fd1b26a2a8aae