fix(non linear font scaling): make FontScaleConverterFactory thread-safe

We use the same technique as CopyOnWriteArrayList to protect the writes
only. Reads can be a bit out of date since they are only used for
caching.

Bug: b/239736383
Flag: android.content.res.font_scale_converter_public
Test: atest FrameworksCoreTests:android.content.res.FontScaleConverterFactoryTest
Change-Id: I2cfefcf7a5c6f56294e87d4c35afc8499c2e7c02