Use custom default font_scale behing flag

When the flag is enabled, we read the default
font scale value from the device_font_scale
property in Settings.

Flag: ACONFIG com.android.window.flags.configurable_font_scale_default DEVELOPMENT
Bug: 316160749
Test: atest SettingsRoboTests:FontSizeDataTest

Change-Id: I03a5ee9a2bd364dca9c75b131832e5f9e868167a