liblog: display valid utf8 characters with 'printable' log format

This started as a change to use mbrtowc() instead of
utf8_character_length() as mbrtowc() does everything that
utf8_character_length() intends to do, but is a libc function. The
change was further intended to add unit tests to ensure that these
functions operate as intended.

It turns out that utf8_character_length() returned an error for the
utf8 characters that I tested, so this also has the side effect of
allowing valid utf8 characters to be printed in the 'printable' log
format, which was the original intention.

Also, print the binary data as hex instead of octal, since it is a
more suitable choice.

Test: new unit tests, existing unit tests, logcat -v printable
Change-Id: I4cc95aee81519411ef47892ca74eb31117c972d2