Avoid arithmetics on localized number string ("0,04") in SpecialWatchlist

Avoid arithmetics on localized number string ("0,04") in SpecialWatchlist

In SpecialWatchlist::cutoffselector(), values like 1/24 or 6/24 are cast
to string via strval(). However, in some locales (e.g. ru_RU.utf8) strval
will return a localized form of the number, e.g. "0,04" instead of "0.04".

This "0,04" is then used in arithmetic operations, where it's treated as 0,
resulting in "0 hours" being shown instead of "1 hour", "2 hours", etc.

Bug: T198501
Change-Id: Iaa4e6170b30a7bb9ce0f22d9d2cc4772b0faa3b8
(cherry picked from commit 6b240c699eb1e965dad3c51107566f1f27ed1887)