Move findPreferredUpstreamType into UNM

This changes the way in which available networks are found. Previously
Tethering asked ConnectivityService for NetworkInfo and checked for
whether or not it was in state CONNECTED.

Here we use the fact that ConnectivityService will not call UNM's
callbacks' onAvailable() methods until the networks in question have
become connected.

Test: as follows
    - built
    - flashed
    - booted
    - runtest framework-net passed
Bug: 29337859
Bug: 32163131

Merged-In: I9937297727aa1a063e499fccd5833ace229b1e8a
Merged-In: Ifa1a34a1fb32149085421a63cb0f2586d2862d6b
Merged-In: Ia215e55b69b856f5511e5d4f852e39fa6c11462e
Change-Id: I97abe225fdd3accb38bd9168f545445b761a90d8
(cherry picked from commit a1d368af2f01e2048240fb730f6cc399fc1cc1ab)
4 files changed