longparas: raise 16bit cursor pos limit to 32bit

Change-Id: I08e4e954d4f07800d23b50d3fc933b1d2984eca0
longparas: raise 16bit Impl_IMEInfos limits to 32bit

Change-Id: I2196119a8205892724f0d8b75a05940666839642
longparas: nStart and nEnd are always overwritten

so simplify to the reality, which conveniently removes some
hard-coded 0xFFFF and STRING_LEN uses

Change-Id: I4fc014a972f6a157c18f8194c410b7cbff03de4a
longparas: these 0xFFFF are STRING_LEN

Change-Id: I9501efae520a994baaf675f70f3570d9ba3233aa