JPtextinput: Dont break emoji characters when cutting strings.

In the LengthFilter make sure that we do not cut the
string so a last emoji character gets broken and replaced
by some garbage character.

We need to check if the last char is the high surrogate and
if so drop the high surrogate also.

Change-Id: I210e7e41aa0761ecbb2b32e3ebf680a04382c287