The keyboard app from the LG G2 has been ported by gerardroid (XDA) thanks.
I tried to install apk for rooted devices and tried to add the lines in build.prob but regardless of whether Root Explorer or Root Browser nothing worked for me.
If there is someone who has the same problem like me. Here is the solution:
1. Download LGG2-keyboard.apk
2. Optional: Download Keyboard Themes: LG_G2_IME_Themes.zip
3. Install the downloaded LGG2-keyboard.apk and themes on your G2
4. Now download Build Prob Editor from PlayStore
5. Open Build Prob Editor and add following lines like the picture below:
ime_extend_row_keyboard=true ime_onehand_keyboard=true ime_split_keyboard=true ime_vibration_pattern=0:20 ro.config.libemoji=libemoji_lge.so ro.lge.capp_emoji=true
6. Having added the lines, exit Build Prob Editor and restart device.
You should now have emoji on your G2-Keyboard
If you have questions ask me