Properly enumerate GX fonts of the same family with divergent styles

Properly enumerate GX fonts of the same family with divergent styles

* doc/emacs/android.texi (Android Fonts): Revise explanation of
GX font replacement during font enumeration.

* src/sfntfont.c (sfnt_replace_fonts_p): If PREV and DESC are of
the same family and both are GX fonts, restrict actions taken to
removing duplicate styles from PREV.  Reported by a user who
tried to install GX versions of Cascadia Mono, which are
distributed as two files providing the bold and italic variation
axes respectively.