Legacy behavior will return success results to multiple startTethering
calls of the same type, even though the link layer can only support one
of those requests at a time (other than tethered AP + LOHS).
For B+, clean up this behavior by only allowing the amount of requests
that the link layer can concurrently support. To do this, we'll
introduce the concept of "fuzzy-matching" such that if a new request
fuzzy-matches an existing request (either pending or serving), this
indicates that the link layer cannot support the new request
concurrently, and we'll fail the new request with
TETHER_ERROR_DUPLICATE_REQUEST.
To keep track of serving requests, we'll store the request and it's
IpServer in RequestTracker when we call IpServer.enable(), and we'll
remove it (indexed by IpServer) when
1) Calling IpServer.onStop()
2) Calling IpServer.unwanted()
3) We detect an IpServer transition to AvailableState with error code
Bug: 216524590
Test: atest TetheringTest RequestTrackerTest TetheringManagerTest
Change-Id: I1e193f3595a48f806fb70fd08630a12ce2631a59
2add9260ef