Refactor netd utility methods to support custom netIds

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
3 files changed