Fix fallbacks for adapted common script

When the platform plugin gives us specific fallbacks for a
specific script, we need to respect this, and load the correct
font family regardless of which writing systems it supports.

This is especially important since the common script is adapted
to match surrounding, proper scripts, so characters such as
digits next to e.g. Hebrew text will be marked as Hebrew. On
stock Android, there is a single Hebrew font, and this would
previously be put in all fallback slots for Hebrew regardless of
what fallback fonts were dictated by the platform plugin. Since
this font does not support the digits, they would show up as boxes.

[ChangeLog][Android] Fixed common characters like digits and
punctuation showing as boxes when positioned next to non-latin
scripts.

Task-number: QTBUG-39377
Change-Id: I1555e208a8ddc587c0bbdbfff1600cafdd9442e9
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>