Remove dead code from NetworkManagementService

The code will never be shipped to a build with SDK_INT before VIC.
Removing all code from conditional branches that will never be executed.

Many APIs touched here can be removed from the AIDL interface later.

Flag: EXEMPT refactor

Test: atest FrameworksServicesTests:NetworkManagementServiceTest

Bug: 339532486
Change-Id: Id3dcc1049864fd1fc5fbb26d90e205183ad3a975