Allow vendor code to link to libandroid_net.

This library contains only the multinetwork API, which is part
of the NDK and has long-term support. It is needed by any native
that wants to use network communications on a different network
than the default network, and thus is necessary for vendor code
such as IMS clients or carrier applications.

Bug: 37527489
Test: marlin builds and boots
Test: MultinetworkApiTest CTS tests passes
Change-Id: I2aa592d1d082aae410173adf2a09f6d1dd8092dc
Merged-In: I2aa592d1d082aae410173adf2a09f6d1dd8092dc