logd: use a std::list<> of values not pointers

This saves 4 or 8 bytes off of each log message for 32 bit or 64 bit
devices respectively.  In practice, this actually saves more, due to
avoiding heap fragmentation.

Averaging over 5 runs of the LogBufferTest.random_messages unit test
(32 bit), this change results in 8k less memory used when 1000 logs
are logged and results in 260k less memory used when 10000 logs are
logged.

Test: check memory usage during LogBufferTest.random_messages
Test: logging unit tests
Change-Id: Ia7953e3c4cb19631ef43bab1deb91bb336bc2520