Fix CBRS DataSwitch sending multiple START events.

 - Fix issue that we send another switch start event again after
 validation done.
 - Fix issue that upon registering default network switch callback,
 as we receive a callback immediately on current network, we wrongly
 record it as switch end. Solution is we remember the expected subId
 and switch reason. Such that only when default network is on expected
 subId, we send switch end event, and with correct switch reason.

Bug: 137872143
Test: manual

Change-Id: I32f311a628c29a592481a6bcd3fe105ff173b22f
Merged-In: I32f311a628c29a592481a6bcd3fe105ff173b22f