[Filesystem] Add validation of decoded UTF-8 characters

[Filesystem] Add validation of decoded UTF-8 characters

_utf8_decode function used to throw a RangeError exception, when an
invalid UTF-8 sequence was converted to a Unicode code point.

Byte sequences, invalid in terms of UTF-8, are now substituted with
an Unicode replacement character.

[Verification] TCT tct-filesystem-tizen-tests and tct-file-cordova-tests
               pass rate on a Z400 mobile device is 100%.
               Decoding was tested manually against numerous problematic
               byte sequences.

Change-Id: If8aefd3434a1b96ead11e36a1db1ddee4f2c3904
Signed-off-by: Pawel Wasowski <p.wasowski2@partner.samsung.com>