Compile native network functionis into libandroid_net.

libandroid includes some local implementations, but depends on many
other libraries to provide those (counting transitive dependencies, a
total of 65). This patch copies the native network functions provided
by android/multinetwork.h header to its own new libandroid_net shared
library. This new library has only one other shared library dependency.

Bug: 32842753
Test: `make` on aosp_arm-eng
Change-Id: Ifbd529d9bf74a8d4f754733cff345729de6b1521