Adds a summary to the (screen) Magnification preference for the main A11y Settings page.

This helps differentiate this from other magnification tools.

Reuses (and slightly modifies) the `magnification_feature_summary`
string which was not used by any existing code. This helps ensure
the existing localizations should be close enough, until L10n
translators have time to update based on the new shorter version.

Bug: 303499185
Test: ToggleScreenMagnificationPreferenceFragmentTest
Test: Open Settings > Accessibility, observe summary
Change-Id: If5ee4f937bbd655b425d4767ad8bbcef08dbe345