[Tether07] Migrate Tethering into module

Now tethering would be run in dedicated service.
TetheringManager is the interface used to communicate with
TetheringService. The new call flow would be: ConnectivityManager
-> ConnectivityService -> TetheringManager -> TetheringService.
Note: the return value of #tether(), #untether() and #setUsbTethering()
APIs would always be no error. Client can use #getLastTetherError()
or #getTetheredIfaces or listen tether state change to check
status of corresponding interface.

Bug: 136040414
Bug: 144742179
Test: -build, flash, boot
      -atest TetheringTests
      -atest FrameworksNetTests

Change-Id: I7e78c0e0a3e70f940a749ba2a39ece7c7ec5b9b3