This commit refactors code and implements a custom device specific string
filter, which does nothing by default.
The implementation in HuamiSupport does the custom emoji conversion.
The setting has been moved from devicesettings_amazfitbip.xml to an extra file
As soon as there is a custom font for Mi Band 2/3/4 it is sufficient to add
"devicesettings_custom_emoji_font.xml" to the list of supported settings
in the appropriate coordinator and everything will work.
