libutils: fix incorrect calculation in utf8_length() method

libutils: fix incorrect calculation in utf8_length() method

The first character of utf-8 could be larger than 128. If use signed char
variable to hold it, it would be treated as negative. That may result in
some unexpected errors.

For example, without this patch, suppose the code is 0xE88888, then
first_char is 0xE8 and converted to int32_t type (0xFFFFFFE8) and
masked with (~to_ignore_mask). The result utf32 is FFF08208
which is incorrect.

Change-Id: I72b355f380865bc375251eb287fc225fd585a115