Add ACTION_TETHER_SETTINGS

Tethering notification needs to launch TetherSettings when user
tapped notification. But OEM can have their own Settings
package. So add ACTION_TETHER_SETTINGS in Settings.java for
launching TetherSettings via intent. Then OEM can launch their
TetherSettings by receiving this intent.

Bug: 149068177
Test: Build, Flash, manual test.
Change-Id: I98f6b50b45f1f8cc1fe6dab7b2592de4611363ce