fix(non linear font scaling): fix crash with certain negative SP values

The crash was due to some old logic that was looking for near matches.
Now we only return exact matches, and let the interpolation deal with
near matches.

Add test to make sure it doesn't crash at any value passed in.

Test: atest FrameworksCoreTests:android.content.res.FontScaleConverterTest

Bug: b/260984829

Change-Id: Ic90d1c5b48862e4f78b90be5926c9ce6d468acf9