Clean up SmsApplication and expose getDefaultSmsPackage

- Clean up usages of hidden AppOpsManager APIs in SmsApplication
- Add unit tests for SmsApplication
- Expose RoleManager#getDefaultSmsPackage for use in SmsApplication.

Bug: 146834818
Test: atest SmsApplicationTest
Change-Id: I4ab49d7f6274389e7de7cd7223cab3c433936044