Make TunInterface more useful.

- Support more than one TunInterface per process by adding some
  randomness to the interface name.
- Export include files so it can be used from other directories.
- Add a method to get the interface index.

Bug: 73032258
Test: netd_unit_test passes, netd_integration_test has no new failures
Change-Id: I0968cd394e18e51aec6b48f07ac626ba2496160f