Add basic implementation of stopTetheringRequest

Add a basic implementation of stopTetheringRequest that simply stops
tethering for that specific type if it is active, or fail with
TETHER_ERROR_UNKNOWN_REQUEST if not. In the future, we will do fuzzy
matching to ensure the correct request is removed.

Bug: 216524590
Test: atest TetheringManagerTest#testStopTetheringRequest
Change-Id: Ib4a660aacdb83e645e39ceb61b7cda45bb24d89d
6 files changed