Properly support handover retry

1. If retrying handover, do not tear down the
   data connection on the source transport.
2. For handover fallback, fail fast the retry instead
   of letting it retry and fail with ON_OTHER_TRANSPORT
   error. This is no behavioral change but optimization
   only.

Fix: 170796126
Test: Manual
Change-Id: I1ee4e638892bfdba19f368c5e2ae7ecaf2c7364c