Migrate to getNetworkSelectionStatus()

isNetworkEnabled() & isNetworkPermanentlyDisabled()
should not be formal APIs. Instead,
getNetworkSelectionStatus() with a set of constants
is sufficient as an API surface, and will be more
maintainable in the future.

Bug: 146046526
Test: atest SettingsUnitTests
Change-Id: If599ce88f68a87fa7186ef96156c2bb1fdc1c2c3