Use @SystemApi definitions for Provisioning where applicable

Two changes:
1) Use public APIs where necessary
2) Fix an issue where we were not setting the WFC roaming
enabled setting correctly when enabling WFC. This was
causing WFC while roaming not to be enabled.

Bug: 109936682
Test: atest FrameworkTelephonyTests
Change-Id: I9e0aba76af55e5423a5f66f3e9a592863a920a49