Magnification: Replace hard-coded constant with SCALE_MAX_VALUE

This allows the magnification settings slider to scale magnification
beyond 8.0 if it is set in a config.

Bug:b/352139715
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:82e67803e71693b98ac41fda6cb2c01e14819cf1)
Change-Id: I94dfba2277d24f7b1367b69f93a5ebb1ceef647a