[SettingsProvider] extend font size scale range

As per requested in b/176940932#comment3 and b/156260178#comment32,
extending the valid font size scale range to [0.25, 5].

Existing CTS tests still pass.

BUG: 156260178
Test: atest android.provider.cts.settings.Settings_SystemTest
Test: atest android.app.cts.ApplicationTest
Change-Id: Icff82d727d63da4353342b0f9a5ca3c2ae1671c1