Previous versions used an empty language tag for LANGUAGE_DONTKNOW with the
"[All]" autocorrection entry and read/wrote from/to arco_.dat file. An empty
language tag otherwise is used for system locale and doesn't convert flawlessly
with the new LanguageTag system. Instead use LANGUAGE_UNDETERMINED with the ISO
639 code 'und' so the file name is "acor_und.dat".

During user profile migration an existing 3/user/autocorr/acor_.dat is copied
to the new 4/user/autocorr/acor_und.dat

Change-Id: I593f24829c6efd58f36e93ebd3385a3c925f7217