Support static address configuration

Application can specify static ipv4 server and client address to setup
tethering and this is one shot configuration. Tethering service would
not save the configuration and the configuration would be reset when
tethering stop or start failure.

When startTethering callback fired, it just mean tethering is requested
successful. Therefore, callers may call startTethering again if
startTethering successful but do not receive following tethering active
notification for a while. Tethering service never actually does anything
synchronously when startTethering is called:
  -startProvisioningIfNeeded just posts a message to the handler thread.
  -enableTetheringInternal doesn't do anything synchronously, it just
  asks the downstreams to get their interfaces ready and waits for
  callbacks.
If tethering is already enabled with a different request,
tethering would be disabled and re-enabled.

Bug: 141256482
Test: -build, flash, boot
      -atest TetheringTests
      -atest CtsTetheringTest

Change-Id: I0399917e7cefa1547d617e688225544c4fc1a231