Make MSCompoundFileReader::readFile platform-agnostic

Make MSCompoundFileReader::readFile platform-agnostic

MSCompoundFileReader::readFile uses iconv to convert information
given in UTF-16 character set with little-endian byte order to
the UTF-8 character set. The input string has no BOM and the byte order
is not explicitly given, causing iconv to try to guess the byte order
based on the host operating system. This causes the method to return
different results for the same file in different environments.

This patch explicitly provides the byte order for the input to be
converted (UTF-16LE) to ensure portability and predictability.

As part of this, move MSCompoundFileReaderTest into the unit test tree.

Bug: T225019
Change-Id: I62154897d303b28c288c3a4f2f5456bedcc81852