Move libandroid_net from Android.mk to Android.bp.

This is the first step of making libandroid_net available to
vendor code. This is a partial cherry-pick of ag/2210044 that
only moves the build target (hence the Merged-In change ID).

Bug: 37527489
Test: marlin builds and boots
Change-Id: Id9e4d13d4e75f6f6381c675c4e9f72e9f4769d22
Merged-In: Ie3c9c3d3c8c75ade36422d1e8ee0a5fe117e9091