Address BiometricManager strings API review feedback

Makes the following changes to the BiometricManager strings API:
- Move string getter methods to a static nested StringProvider class
- Add a BiometricManager method to get a StringProvider instance
- Document when null may be returned and how (not) to interpret it

Test: atest BiometricManagerTest
Test: Manually logged strings in biometric test app

Bug: 182793560
Change-Id: I27862bd5f67ea291f166a7ac91b03c7c432da64f