Add the defaultNetwork element to the netstats.proto.

This is in its own commit because AOSP has diverged from internal
master.

Bug: 35142602
Test: builds, boots
Change-Id: I75fc85b961bc919e3d6c2adce6d6861c6f2c8379
Merged-In: I75fc85b961bc919e3d6c2adce6d6861c6f2c8379