Fixes a11y captioning B&R validators for font scale and locale.

- Font scale: can go down to 0.25, so update the validator range
- Locale: Use a "loose" locale validator which allows subset
          matching against the installed locale set, since this
          setting may not include locale details like Script
          while still representing a valid locale for the device.

Fix: 384862163
Change-Id: I05f14666a7013ac9fadee86787bf688c8b19c945
Test: atest SettingsValidatorsTest
Test: perform B&R of these settings; see b/384862163  comment 10
Flag: EXEMPT low-risk bugfix