Remove unused methods from NetworkManagementService.

More methods are unused, but they are marked @UnsupportedAppUsage
and therefore aren't removed by this patch.

Also address warnings :
• member can be final
• deprecated NetworkStack permission check methods
• redundant public modifier

Test: TH
Bug: 64955351
(cherry picked from commit eec9f31d85fc090722015830a9ad87f9f29b035f)
(cherry picked from commit 5956c8058c468d11d37e2dbd0c847da368ac4cd5)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:158469f5201bfe432da324272400631c91bc78fc)
Merged-In: Ib0d4e8c6a063ed57c5a790eab3868ed4fe9cf096
Change-Id: Ib0d4e8c6a063ed57c5a790eab3868ed4fe9cf096