libutils/Unicode.cpp: Correct length computation and add checks for utf16->utf8

Inconsistent behaviour between utf16_to_utf8 and utf16_to_utf8_length
is causing a heap overflow.

Correcting the length computation and adding bound checks to the
conversion functions.

(cherry picked from commit c4966a36)
(changed code for safetynet logging due to lack of sstream and string in klp)

Change-Id: If50d59a91a13fddbff9a8fff0d3eebe57c711e93
Bug: 29250543