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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d81dd9a4c4c13da2da800866d2ed9ae7a0f4d41e)
Merged-In: Id3dcc1049864fd1fc5fbb26d90e205183ad3a975
Change-Id: Id3dcc1049864fd1fc5fbb26d90e205183ad3a975