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
Merged-In: Ib0d4e8c6a063ed57c5a790eab3868ed4fe9cf096
Change-Id: Ib0d4e8c6a063ed57c5a790eab3868ed4fe9cf096