Start L2capNetworkProvider in CS

Starts L2capNetworkProvider in ConnectivityService. Alternatively, this
could be moved to ConnectivityServiceInitializer instead in a follow up.

When L2capNetworkProvider is created, it starts a handler thread, so
makeL2capNetworkProvider must return null in tests as otherwise the
thread leaks.

Test: atest NetworkReservationTest
Change-Id: I35c14815b1a71f0739c1bf63ad466763d611434c
5 files changed