This is a no-op refactoring which:
1. Updates netd utility methods to handle custom netIds.
2. Rename internal methods to reflect the functionality.
3. Import constants from TetheringManager to improve
readability.
This refactoring is a prerequisite for future changes that will
utilize a new netId for routing instead of LOCAL_NETWORK_ID.
Test: atest NetdStaticLibTests TetheringTests CtsTetheringTest
Test: adb shell ip ru && adb shell ip ro show table local_network
Bug: 349487600
Change-Id: I1f3a68d24852b6bd85ce7be2ec58746e49b67636
04783040d5