fix(magnification suw): followTyping preference should be hidden in magnification suw page

In ag/26349366 we wrapped the follow typing prefernce creation into new
method, but in the new method we didn't assign the created preference to
mFollowingTypingSwitchPreference. So
ToggleScreenMagnificationPreferenceFragmentForSetupWizard can not access
the preference object to hide it. Therefore, as a short-term solution we
assign the created preference to mFollowingTypingSwitchPreference. For a
long-term plan we should not create a preference and hide it in suw,
instead we don't need to create that preference. (tracked in b/335788167)

Bug: 335788769
Flag: NONE
Test: manually
Change-Id: Ideef93127343b7d1105a63006d343cd5ef66de08