feat(non linear font scaling): make FontScaleConverter APIs public

This allows other UI frameworks (Compose, Flutter, etc) to use the
conversions directly without needing a Context or display density.

Moved some tests over to CTS.

Bug: 239736383
Flag: android.content.res.font_scale_converter_public
Test: atest FrameworksCoreTests:android.content.res.FontScaleConverterActivityTest
  && atest FrameworksCoreTests:android.content.res.FontScaleConverterTest
  && atest FrameworksCoreTests:android.content.res.FontScaleConverterFactoryTest
  && atest cts/tests/tests/content/src/android/content/res/cts/FontScaleConverterFactoryTest.kt

Change-Id: I824a9e7dbd8489bf316064eadd62c1c082a12de5