Replace usage of NOT_FIXED_DEC with DECIMAL_NOT_SPECIFIED.
Currently these constants are used interchangeably which
is confusing. Even if NOT_FIXED_DEC is used more often,
DECIMAL_NOT_SPECIFIED is kept since it referenced in the
documentation of Item::decimal and is an easier name to understand.

Change-Id: Idd0418b28a0e119d35d2ba37bd82368765ec085e