Pick default interface for upstream.

With multiple upstream interfaces active and connected,
we should choose the default interface for upstream.
CRs-fixed: 461266

Change-Id: Ia3bc32300a1f77f46237994d0e38e9c9886f4bea
Tethering: fix NPE when retrieving active interfaces

When in airplane mode getActiveNetworkInfo() may return null.

Change-Id: Icba37abb51b9378c8af73d9f6587e04c588dd85e
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>